I already hopeknew it would be hard, but not that hard.

(or) I expected it to be hard, but not that hard.

From now on the students of each class (there are two of them) will be placed by their choice.

From now on, students in each class (there are two of them) will be able to choose what they study.

The choice that most of my friends and I didmade will probably make us study in the afternoon, but two of themstudents made a different choice.

to make a choice = to choose

Although we won't see each other every day anymore, I know we will meet often, so it didoesn't makde me sad.

What made me sad was to say goodbye to someone I was starting to build a relationship with., but at least I did ask her to hang with me so I know it wasn't an ending's not over yet.

I'm not the crying type, but when one friend hugged me while she was crying, the strength of the hug made me want to cry - she will be in the same class as me, but we were sad because of the others that won't .

I already hope it would be hard, but not that hard.

Consider also:
I had already hoped it would be hard but not that hard.

I think the past perfect tense is probably more appropriate here. If I understand your sentence properly, it seems that you are speaking about an event in the past (the last day of school), and at that time you had already beforehand hoped that that event would be hard (so the hoping started even further in the past). This sort of thing usually needs the past perfect tense.
Past simple: I hoped.
Past imperfect (continuous action in the past): I was hoping.
Past perfect (already completed action in the past): I had hoped.

Please don't worry too much about this. It will get easier with practice. Keep going with LangCorrect and people will help you to get things right!
If I have misunderstood what you were actually trying to say, I apologise!

Basically, there are two paths in high school, soand you have to choose one.

The choice that most of my friends and I didmade will probably make us study in the afternoon, but two of them made a different choice.

Although we won't see each other every day anymore, I know we will meet often, so it didn't make me sad.

A comma (,) is probably needed here just to help with readability and understanding.

What made me sad was to saying goodbye to someone I was starting to build a relationship with., but a At least I did ask her to hang out with me, so I knoew it wasn't anthe ending yet.

I've made a few suggestions to help the text flow better.

I'm not the crying type, but when one friend hugged me while she was crying, the strength of the hug made me want to cry - s. She will be in the same class as me, but we were sad because of the others that won't .

A few more suggestions!