Today's weather was extremely harsh in my town.

The apostrophe here marks a possessive function (e.g. "tomorrow's earnings").

We hadwere at, like, 38 degrees Celsius and ahad really humid air.

Temperature in terms of weather isn't necessarily something that you own, but rather a state of being that can affect other things (e.g. people, plants, and animals). Also, I assume you used "like" as a filler word or qualifier, so that's why I surrounded it with commas.

We had to go to school as usual, with the slight advantage, that outr 90-minute lessons were shortened to 60 minutes.

If you're using a period of time as an adjective, it's best to hyphenate it like I did. Also, I assumed that "out" was a typo for "our," so that's why I rewrote it.

I mean, don't get me wrong, i. It's nice to get some more free time, but today was still a long day for me.

For simplicity's sake, I think it'd be best to split up long sentences like this into shorter ones.

Normally, I always have to be at school till 4pm:00 P.M. on Thursdays.

Either "4pm" or my correction can work for this.

This time, it was, like, 3pm:00 P.M. when I left school.

Same goes for "3pm" or my correction here.

That's still a really long time, considering, that you are sitting with at least 20 other people in one big room, without air conditioning, and the sun is shining in your classroom.

I think you figured that "air conditioning" was all one word in English, like "Klimaanlage" in German. Be sure to split up compound words like this going forward.

After that i, I had to run to my school bus, which also had no AC and e. Every seat was taken.

Same goes for "school bus" in English compared to "Schulbus" in German.

If it wasn't for the extreme temperatures, this wouldn't be an issiue, but the 40 -minute drive home felt like another 2 hours at school.

You could also split this up into two sentences by deleting "but" and capitalizing the second "the," but I won't make any more corrections here because this is a coherent thought with a logical connection to it.

Today's weather was extremely harsh in my town.

We had likeIt was 38 degrees Celsius and athe air was really humid air.

We had to go to school as usual, with the slight advantage, that outr 90 minute lessons were shortened to 60 minutes.

That's still a really long time, considering, you are sitting with at least 20 other people in one big room, without air conditioning, and the sun is shining in your classroom.

After that i, I had to run to my school bus, which also had no AC and every seat was taken.

Today's weather was extremely harsh in my town.

We had like 38 degrees Celsius and a really humid air.

We had to go to school as usual, with the slight advantage, that outr 90 minute lessons were shortened to 60 minutes.

After that iI had to run to my school bus, which also had no AC and every seat was taken.

If it wasn't for the extreme temperatures, this wouldn't be an issiue, but the 40 minute drive home felt like another 2 hours at school.