xiàkè yǐhòu wǒ gēn tóngxué yìqǐ huí sùshè xiūxi.

Both word orders are possible, but they have slightly different emphasis.

1. 下课以后，我跟同学一起回宿舍休息。

   • Time phrase 下课以后 comes first.
   • This is very natural in Chinese: Time – Subject – (Other info) – Verb – Object.
   • It emphasizes when the action happens.

2. 我下课以后跟同学一起回宿舍休息。

   • Subject 我 comes first, then the time phrase.
   • Also correct and common.
   • Slightly more focus on I as the topic.

So:

• Both are grammatically fine.
• Everyday speech and writing often put general time expressions like 下课以后, 明天, 晚上 before the subject, but putting them after the subject is also common:
  • 明天我去北京。 / 我明天去北京。
  • 下课以后我回家。 / 我下课以后回家。

In this sentence, 跟 means with:

• 我跟同学一起回宿舍休息。
  I go back to the dorm to rest with my classmates.

Both 跟 and 和 can mean with when connecting people:

• 我跟同学去。
• 我和同学去。

Differences in usage:

1. Spoken vs written:

   • 跟 is very common in spoken Chinese and sounds slightly more casual.
   • 和 is also used in speech but may sound a bit more neutral or written, depending on region.

2. Patterns:

   • 跟 is more common in patterns like:
     • 跟……一起…… (do something together with …)
       我跟他一起吃饭。
     • 跟……说 (say to …)
       我跟老师说。
   • 和 often appears in:
     • A 和 B (A and B)
       我和同学都是学生。

In this sentence, using 跟 is very natural, and most native speakers would say 跟 here. Using 和 instead would still be correct:
下课以后我和同学一起回宿舍休息。

You are right that they overlap in meaning, but they play different roles:

• 跟同学 = with classmates / together with classmates (shows the companion)
• 一起 = together (describes how the action is done)

In 我跟同学一起回宿舍休息:

• 跟同学: tells you who I am with.
• 一起: emphasizes we are doing the action together.

Can you drop one?

1. 我跟同学回宿舍休息。

   • Still understandable.
   • Means I, along with my classmates, go back to the dorm to rest.
   • The idea of together is implied but not strongly emphasized.

2. 我和同学一起回宿舍休息。

   • Keep 一起, still clear.
   • Emphasizes the togetherness more.

3. 我一起回宿舍休息。

   • Odd, because 一起 needs someone to be together with, but that someone isn’t mentioned.

So the combination 跟 + person + 一起 is a very natural, common pattern:
我跟朋友一起吃饭。
他跟家人一起住。

们 is the plural marker for people, but it is not always required, even when the meaning is plural.

• 同学 can mean:
  • a classmate
  • classmates (in general / some classmates)

In many contexts, Chinese leaves plurality to be understood from context instead of marking it:

• 我有同学在美国。 = I have a classmate / some classmates in the US.
• 下课以后我跟同学一起回宿舍休息。
  Obviously we’re talking about several people going together, so 同学 is understood as plural.

When to use 同学们:

• When you want to address the whole group explicitly, often as a teacher or speaker:
  • 同学们，好！
  • 老师跟同学们说：……

In this sentence, 同学 without 们 is more natural. 同学们 would sound like “the whole class” and often implies a teacher talking to students, which is not the case here.

In Chinese, the possessive 的 is often omitted when the relationship is clear and close, especially with:

• family: 我妈妈, 我哥哥 (here 的 is often dropped)
• close relations or obvious roles: 同学, 同事, 朋友 often don’t need 的 if the owner is already clear.

In this sentence:

• The subject is 我.
• When you say 同学 in a sentence where 我 is the subject, it’s naturally understood as my classmates.

If you say 我的同学, it’s still correct, but:

• It sounds a bit more specific, as if you are contrasting my classmates with someone else’s classmates.
• In this neutral context, 跟同学 is simpler and more natural.

Both 回 and 去 talk about movement, but they focus on different ideas:

• 去 = go (to) some place (neutral direction, just moving away from here)
• 回 = go back / return (go back to a place you belong to or came from)

In a school context, a 宿舍 (dorm) is understood as your living place, a place you return to, so 回宿舍 is very natural:

• 回宿舍 = go back to the dorm.
• 去宿舍 = go to the dorm (just a neutral “go”, may or may not be your own dorm).

You could say:

• 下课以后我去宿舍休息。
  This is grammatically correct, but it doesn’t highlight the idea of going back home / back to where you stay.
• 下课以后我回宿舍休息。
  More natural because dorm = “home base” at school, something you return to.

Putting 了 after 回宿舍 in this sentence is not natural:

• 下课以后我跟同学一起回宿舍休息。 ✅
• 下课以后我跟同学一起回宿舍了休息。 ❌ (sounds wrong/awkward)

Reasons:

1. Verb sequence pattern
   The sentence uses a simple sequence of actions:

   • 回宿舍 (go back to the dorm)
   • 休息 (rest)

   In many cases, Chinese just lists verbs in order without adding aspect markers:
   回家吃饭 (go home and eat), 去图书馆看书 (go to the library [and] read).

2. When do we use 了 here?
   You could add 了 at the end of the sentence to show that this already happened:

   • 下课以后我跟同学一起回宿舍休息了。
     = After class, I (have) gone back to the dorm with my classmates to rest (it’s completed).

   But putting 了 right after 回宿舍 and then adding 休息 is odd in this structure.

So:

• No 了: neutral statement of what (typically) happens.
• 了 at the very end: emphasizes that this happened already.
• 了 after 回宿舍 but before 休息: not natural here.

下课 means class finishes / to get out of class.

Usage:

1. As a verb (intransitive):

   • 我们三点下课。
     Our class ends at three.
   • 老师还没下课。
     The teacher hasn’t ended the class yet.

2. As part of a time phrase:

   • 下课以后 = after class ends.
   • 下课以后休息一下。 = Rest for a bit after class.

Contrast with:

• 上课 = have class / be in class / start class
  
  • 我们八点上课。 We start class at eight.

So in 下课以后我跟同学一起回宿舍休息:

• 下课以后 = after the class is over, not during class.
• The sentence describes a routine or a typical event after class.

No. 课下以后 is not a standard way to say after class.

• 下课以后 is the normal, idiomatic expression meaning after class finishes.
• 课下 sometimes appears in very specific contexts meaning outside of class time, but 课下以后 is not commonly used and would sound strange in this basic meaning.

So use:

• 下课以后 = after class (ends).
  Not 课下以后.

以后 is quite flexible, but not every position works.

Natural positions in this sentence:

1. 下课以后我跟同学一起回宿舍休息。 ✅
   (very natural)

2. 我下课以后跟同学一起回宿舍休息。 ✅
   (also natural)

3. 下课了以后我跟同学一起回宿舍休息。 ✅
   (also fine; 了 emphasizes “after it has finished”)

Positions that are not natural or change the meaning:

• 我以后下课跟同学一起回宿舍休息。
  This means from now on, after class I will go back to the dorm with classmates to rest.
  Here 以后 = from now on / in the future, modifying the whole sentence, not just 下课.

• 下课我以后跟同学一起回宿舍休息。
  This is awkward and unnatural; 以后 is in a strange position.

So, if you just want after class, the best forms are:

• 下课以后……
• 我下课以后……
• 下课了以后…… (more explicit about the completion)

The word 一起 is pronounced yìqǐ (4th tone + 3rd tone), not yīqǐ.

General rule for 一 when it’s a separate word meaning “one”:

• Before a 4th tone, it usually changes to 2nd tone: yí.
  • 一个 → yí gè
  • 一半 → yí bàn
• Before 1st, 2nd, 3rd tones, it usually becomes 4th tone: yì.

However, in fixed words like 一起, the tones are fixed as part of the word:

• Dictionary form: 一起 = yìqǐ.
• Here 一 is not acting like the number one; it’s part of a compound meaning together.

So you should simply remember 一起 = yìqǐ.