On weekdays, I wake up at 7 am,. after a quick wash and snack for breakfast, mostly isusually an egg, a bottle of warm milk boiled with oatmeal and a piece of bread.

Around 20 minutes later, I drive to school.

if you drive with your own car, then this is correct. if you ride on a bus/car with someone else driving it, it would be “ Around 20 minutes later, I take a ride to school.”

If there were no traffic jams, I would arrive in the classroom at around 7:40.

On Saturdays and Sundays are great days for me.

On weekdays, I wake up at 7 am, a.m. After a quick wash and snack, I have breakfast, mostly is an egg, a bottle of warm milk boiled with oatmealwhich is often an egg, oatmeal cooked in milk, and a piece of bread.

1. There are two ways to write "ante meridiem": either a.m. or AM. I personally write 7 AM in my own casual writing, but 7 a.m. is recommended by most style guides.
2. I wasn't certain whether the oat/milk component of your breakfast is a beverage made mostly of milk or a food made mostly of oats. My suggestion assumes you are referring to something more like food. Cereal grains (e.g. oats) boiled in water or milk can also be called "porridge" in English.

If there weare no traffic jams, I would arrive inat my classroom at around 7:40 a.m.

1. The simple present tense is used for writing about repeated actions and habits. Since you are describing your usual routine, continue writing in present tense in this sentence.
2. Here's a potentially helpful guideline: Use "arrive in" when your destination is a town, city, or country. Use "arrive at" when your destination is a building or a part of a building.

In the afternoon, I generally leave school at 4:00 p.m.

On Saturdays and Sundays are great days for me.

ALSO POSSIBLE: My Daily Routine

On weekdays, I usually wake up at 7 am, after a quick wash and have a snack breakfast, mostlywhich is often is an egg, a bottle of warm milk boiled with oatmeal and a piece of bread.

If there were no traffic jams, I would arrive inreach my classroom at around 7:40.

On weekdays, I wake up at 7 am. About 20 minutes later, after a quick wash and sna(quick) breakfast, mostly is—usually an egg, (a bottle of) warm milk boiled with oatmeal and a piece of bread.—I drive to school.

Your original wording means that you wash and have breakfast BEFORE you wake up. :-))

Also, “a bottle of warm milk boiled with oatmeal” means that the oatmeal and milk are served in the bottle. You don’t mention the bottle in your Chinese text, so I’m not sure what you mean.

If there were no traffic jams, I would arrive in the classroom at around 7:40.

On Saturdays and Sundays are great days for me.

On weekdays, I wake up at 7 am, after a quick wash and snacka breakfast, snack, which is mostly is an egg, a bottle of warm milk boiled with oatmeal and a piece of bread.

If there were noaren’t any traffic jams, I would arrive in classroom at around 7:40.

On Saturdays and Sundays are great days for me.

On weekdays, I wake up at 7 am, a. After a quick wash and snackI have a breakfast snack , mostly is an egg, a bottle of warm milk boiled with oatmeal and a piece of bread.

If there weare no traffic jams, I wouldusually arrive in the classroom at around 7:40.

When you are talking about regular routines, we would normally use the present tense.

On Saturdays and Sundays are great days for me.