Today was the end of the week end, so I went to high school and I saw my friends.

Weekend is just one word!

Place a comma before 'so' because both clauses of the sentence are independent!

There's no need to say 'I' again after 'and', since you've already said it before.

I started with a sSpanish class and I had a test.

It's more natural in English to just say the name of the class without putting 'a' before.

Remember, language names are proper nouns, so do capitalise them!

Again, there is no need to repeat 'I' if you have said it before.

Also, please keep in mind:

If you said, "I started with Spanish class, and I had a test", you need a comma since both clauses are independent.

However, "I started with Spanish class and had a test" doesn't need one.

I think I will haveget a great grade because I learnt my lessonsstudied, and it was quite simple.

Say 'get' a grade instead of 'have' because the grade is something you're receiving!

'Learn' is usually in the context of learning new information. 'Study' or 'revise' means to review what you have already learnt!

You already used a conjunction ("because"), so place a comma before 'and'!

"Quite" is used really well here! Sounds super natural.

Afterwards, I had sports class, but my teacher wasn't there.

If you're gonna say "After", you need to say after what, i.e "After Spanish I had sports class"

'Afterwards' is used when you already mentioned what you did in a previous sentence, so saying 'afterwards' refers back to what you said there without repeating it.

I recommend using 'afterwards' in this case so that the text doesn't become repetitive.

Saying 'here' means where you are currently. Since you're not there anymore, say 'there'.

'Afterwards' is a sentence starter, so you need a comma after it.

Since both clauses are independent, you need a comma before 'but'.

So,I was with a friend, w. We wanted into go to the gym (to work out) because he wenthad been going there for 2 weeks.

This is a really wrong sentence, so I broke it up!

It is not clear to me why you used 'So' in the beginning. Unless the reason you went to the gym was because your sports teacher wasn't there, don't start with 'so' since it implies causation. If you did, that's fine.

Saying 'went' is in the simple past tense, which implies your friend stopped going.

Quick recap on tenses:

Simple past tense - the action happened in the past and ended in the past. For example: "I lived here."

Present perfect tense - the action happened in the past and continues in the present. There is more emphasis on the result of the action. For example: "I have lived here for the past 5 months."

Present perfect continuous tense - the action happened in the past and is definitely continuing in the present. More emphasis on the action itself. For example: "I have been living here for the past 5 months."

Depending on what you meant, go for one of these!

Do keep in mind that using brackets here is fine, but in more formal writings, they're too casual.

But we couldn't because I had to be 16 and, but I am fifteen.

There's actually no hard rule that you can't start a sentence with a coordinating conjunction like 'but'. I just want to inform you, though, that a lot of teachers and professors tend not to like it (for no good reason at all).

I also recommend using 'but' here since you're contradicting your other clause.

And the worse in thatwhat's worse is that I will be sixteen tomorrow.

"And what's worse" is a very common English phrase, which I think is what you're going for.

There's no need to say 'in that' because the reader knows what you're referring to!

But I'm going to sign up infor another gym close to my friend's gym, so we will can go there together.

Use 'sign up for' when you're signing up for a thing in your sentence ('I signed up for the soup kitchen')
Use 'sign up to' when you're describing the action ('I signed up to be a volunteer')

Today was the end of the week end so I went to high school and I saw my friends.

I started with a sSpanish class and I had a test.

I think I will haveget a great grade because I learntworked hard in my lessons and it was quite simple.

"I will have" makes sense but isn't very natural :)

"I learnt my lessons" makes it sound like you've made mistakes before but now you get it whereas "worked hard" just shows you did stuff :)

After that, I had sports class but my teacher wasn't there.

"Here" is in reference to right where you are right now whereas "there" references where you were before :)

So, with a friend, we waented in back to the gym (to work out) because he wentas he's been going there for 2 weeks.

"wanted" doesn't work here as you can't "want the gym" as the gym is a building, it implies you want to own the gym

"Because" works here but "as" sound more natural

"he went there for 2 weeks" implies he isn't anymore but with you guys going again, I'm guessing he's still going and so I changed it to mean that he's gone there for two weeks and is still continuing to do so :)

And the worse in thatt thing is that I will be sixteen tomorrow.

But I'm going to sign up into another gym close to my friend's gym so we will cancan still go there together.