míngtiān bā diǎn kāishǐ shàngkè, dàjiā yào zhǔnshí dào jiàoshì.

In Mandarin, time expressions often come early in the sentence, usually before the main verb.

So:

• 明天八点开始上课
• literally follows the pattern [time] + [action]

This is very normal Chinese word order. English often says Class starts at 8 tomorrow, but Mandarin commonly prefers Tomorrow at 8, class starts.

A useful pattern is:

时间 + 动作

Examples:

• 明天去学校 = go to school tomorrow
• 晚上吃饭 = eat dinner in the evening

Mandarin usually does not need a preposition like at before a time expression.

So English:

• at 8 o’clock tomorrow

becomes Chinese:

• 明天八点

You can think of Chinese time expressions as functioning more directly in the sentence.

Compare:

• 我明天去。 = I’m going tomorrow.
• 我们三点见。 = We’ll meet at 3.

Adding 在 before a specific time is usually unnecessary here.

八点 means 8 o’clock.

The word 点 is used for clock hours.
You can also say 八点钟, and both are natural.

• 八点 = 8:00
• 八点钟 = 8 o’clock

In everyday speech, 八点 is very common and slightly shorter.
So in this sentence, 明天八点 simply means tomorrow at 8.

开始 means to begin/start, and 上课 means to have class / attend class / class is in session, depending on context.

So:

• 开始上课 = to begin class / class begins

Here, 开始 is followed by another verb phrase, 上课.

This is a common pattern in Mandarin:

• 开始吃饭 = start eating
• 开始工作 = start working
• 开始学习 = start studying

So 八点开始上课 means that at 8 o’clock, class begins.

It can vary by context, which is why learners often find it tricky.

Most commonly:

• for students, 上课 = to attend class / have class
• for teachers, it can also mean to teach a class

In this sentence, 开始上课 is best understood as class begins or we start class.

So it is not mainly emphasizing the physical act of going somewhere.
That idea appears later in the sentence with 到教室 = arrive at the classroom.

Mandarin often omits subjects when they are obvious from context.

In English, we often need something like:

• The class starts at 8 tomorrow
• We start class at 8 tomorrow

In Chinese, if the context is clear, it is very natural to simply say:

• 明天八点开始上课

The listener understands that this refers to the class, school, or everyone involved.

This kind of subject omission is very common in Mandarin.

Yes. 大家 means everyone or all of you/us, depending on context.

In this sentence:

• 大家要准时到教室 means
• everyone should/must arrive at the classroom on time

It is a very common word in announcements, instructions, and classroom speech.

Examples:

• 大家好 = Hello everyone
• 请大家安静 = Everyone, please be quiet

Here 要 expresses requirement, expectation, or necessity, not just future time.

So:

• 大家要准时到教室 means
• everyone needs to / should / must arrive at the classroom on time

In other contexts, 要 can mean different things, such as:

• want to
• be going to
• need to / must

Here the best interpretation is must/should because it sounds like an instruction or rule.

准时 means on time / punctual.

In 准时到教室, it describes the action 到教室:

• arrive at the classroom on time

So the meaning is not just be punctual in a general sense, but specifically arrive at the classroom on time.

You can think of the structure as:

• 准时 + verb phrase

Examples:

• 准时到 = arrive on time
• 准时开始 = start on time
• 准时交作业 = hand in homework on time

Because 到 means to arrive at / reach, while 在 means to be at / in.

So:

• 到教室 = arrive at the classroom
• 在教室 = be in the classroom

The sentence is telling people to get to the classroom on time, not just describing where they are.

Compare:

• 大家要准时到教室。 = Everyone must arrive at the classroom on time.
• 大家都在教室。 = Everyone is in the classroom.

Strictly speaking, the sentence says:

• class starts at 8:00
• everyone should arrive on time

In real life, this usually implies that people should be in the classroom by 8:00, not walking in after class has already started.

So the practical meaning is:

• Be in the classroom by 8 o’clock.

Chinese 准时 means meeting the required time exactly, but in a classroom situation it usually carries the idea of not being late.

Yes, that is possible, but it shifts the structure a little.

• 明天八点开始上课，大家要准时到教室。 = Tomorrow at 8 class begins; everyone must arrive at the classroom on time.

• 明天八点大家开始上课。 = Tomorrow at 8 everyone starts class.

Your version is more direct about 大家 being the subject of 开始上课.
The original sentence sounds a bit more like an announcement, with two linked ideas:

1. Class starts at 8 tomorrow
2. Everyone must arrive on time

Yes, very common. It sounds natural for a teacher, school, or organizer.

It uses several features typical of announcements:

• time first: 明天八点
• event: 开始上课
• instruction: 大家要...
• clear requirement: 准时到教室

So it has the tone of:

• Tomorrow class starts at 8. Everyone must be in the classroom on time.

This is a very useful model for formal or semi-formal spoken and written Chinese.