Learn Hardcore Korean: sueop junge jipjungharyeogo noryeokhaeyo.

중에 here means “during” (literally “in the middle of”). So 수업 중에 = “during class.”
동안 emphasizes the length of time. With events like “class,” it’s more natural to say 수업하는 동안 or 수업 시간 동안, rather than bare 수업 동안.
때 means “at the time (when).” 수업 때 is casual and common in speech.
Summary:
수업 중에 = during class (activity in progress)
수업(을) 하는 동안 / 수업 시간 동안 = for the duration of class
수업 때 = when it’s class (colloquial “in class”)

Can I say 수업에서 instead of 수업 중에?

수업에서 is usually “in class (at class) [place/context]” and often introduces what happened in class: 수업에서 시험을 봤어요 (We took a test in class).
수업 중에 focuses on the time period during which something occurs.
In your sentence, 수업 중에 is the best fit because you’re talking about what you do during that time block (you try to concentrate).

Is 수업 중 without 에 okay? When do I use 중 vs 중에?

수업 중 is fine as a noun phrase or on signs/labels (e.g., “수업 중” = “Class in session”).
In running sentences as an adverbial (“during class”), 수업 중에 is more natural: 수업 중에 전화를 받지 마세요.
So: signs/labels or headlines → 수업 중, regular sentence adverbial → 수업 중에.

What does -(으)려고 do in 집중하려고?

-(으)려고 marks purpose/intention: “in order to / to.”
Pattern: V-(으)려고 + main verb (often movement, attempt, or intention verbs).
In this sentence, 집중하려고 노력해요 = “I make an effort in order to concentrate.”
Formation:
After a vowel: 가려고, 하려고, 배우려고
After a consonant: 먹으려고, 찾으려고
Some irregulars: 돕다 → 도우려고, 듣다 → 들으려고, 만들다 → 만들려고

I’ve seen people write 할려고. Is that correct?

Standard Korean is 하려고 (not “할려고”).
The error happens by analogy with forms like 할 거예요. Remember: attach -(으)려고 directly to the verb stem, so 하- + 려고 → 하려고.

Isn’t 노력하다 already “to try”? Why add -(으)려고 before it?

노력하다 means “to make an effort.”
V-(으)려고 노력하다 means “to make an effort in order to V,” which explicitly states the goal of your effort.
Nuances:
집중해요 = I concentrate (statement of fact/habit).
집중하려고 해요 = I try/intend to concentrate (lighter, intention).
집중하려고 노력해요 = I make an effort to concentrate (emphasizes effort/striving).

Could I say 집중하기 위해 노력해요 instead? What’s the difference from 집중하려고 노력해요?

Yes. V-기 위해(서) 노력하다 is fully natural and even very common with 노력하다.
-기 위해(서) is a bit more formal/written; -(으)려고 is neutral and very common in speech.
Meaning is the same: both mean “I make an effort to concentrate.”

Is it okay to say just 수업 중에 집중해요?

Yes. That means “I concentrate during class,” asserting that you actually do it.
If you want to convey effort more than guaranteed success, 집중하려고 노력해요/해요 is better.

Why is there no subject like 저는?

Does 집중하다 take an object? Should it be 수업에 집중하려고 노력해요?

집중하다 can be intransitive (just “to concentrate”), or you can mark what you focus on with -에:
- Intransitive: 요즘 잘 집중해요.
- With target: 수업에 집중하려고 노력해요. (“I try to focus on the class.”)
  Both are fine; adding -에 specifies the focus.

How would I make this more formal or more casual?

Is 집중을 하려고 노력해요 okay?

You may hear N(을/를) 하다 with many Sino-Korean nouns (e.g., 공부(를) 하다).
집중(을) 하다 is used, but the fused verb 집중하다 is more concise and typically preferred in careful writing.
So 집중하려고 노력해요 is cleaner.

Pronunciation tips for this sentence?

집중 is typically pronounced with a tensed ㅈ: [집쭝].
수업 중에 is [수업 중에] (smooth linking).
하려고 is [하려고], not [할려고].
노력해요 is [노려캐요] (ㅎ + ㅐ can sound like [캐] due to aspiration on the ㅋ-like onset in fast speech).
Romanization (Revised): su-eop junge jipjung haryeogo noryeokhaeyo.

Can 중에 also mean “among”? Is that the same 중에?

Yes, same word 중(에) “middle/among.”
세 과목 중에 어떤 게 좋아요? = “Among the three courses, which do you like?”
In your sentence, 수업 중에 uses the “during/in the middle of (time)” sense, not the “among” sense. Context disambiguates it.

Is 수업 도중에 or 수업 시간에 okay?

수업 도중에 = “in the middle of class,” a bit more formal/literary than 수업 중에.
수업 시간에 = “during class time,” perfectly natural.
All three work; 수업 중에 is the most neutral, everyday choice.

Where can I place 수업 중에 in the sentence?

Most natural: put it early, before the main predicate: 수업 중에 [집중하려고 노력해요].
After the topic/subject: 저는 수업 중에…
End placement like 집중하려고 노력해요, 수업 중에 sounds marked/less natural in neutral speech. Keep it before the verb phrase.