re: @needs native check
I assume this is a reasonable conveying of the meaning of the Japanese, and certainly I believe I can parse it correctly. That said it is not remotely native, and doesn't seem grammatically correct. Without understanding the Japanese, I wouldn't want to suggest a correction.

でも模試の日は部活が休みになって遥と帰れるし、感謝してるけど、僕は。

Here are some clues to the Japanese,
drawn from Jim Breen's online dictionary.

模試 【もし】 (n) (abbr) (See 模擬試験) mock examination; trial examination

So: "... on the day of the mock exams ...".

部活 【ぶかつ】 (n) club activities

休みになって
appears to mean "... was going on break".

遥 = Haruka, a person's name

遥と帰れる
I think it means, "I can return with Haruka".

So maybe this:

"But it's because of the club going on break
on the day of the mock exams that I can return with Haruka,
I'm thankful for that."

To me it is natural and understandable in the sense of mock exams, and some sort of club (dance/night, or activity). The only suggestion I have is that it is two sentences. "I am grateful for that" seems a separate sentence.

I would suggest this:

"But since there's no club activity on the day of the mocks, I walk back with Haruka, and I'm thankful for that."

or

"But since there's no club activity on the day of the mocks, I walk back with Haruka. I'm thankful for that."

I suggest: "But on the day of the mock exams, there are no club activities, so that means I can walk back home with Haruka. And I'm grateful for that." What do you think?

That sounds fine.

Could you remove the @needs native check tag? I'm a little bit disappointed that my new status doesn't allow me to remove these tags.

A while ago, we created the following tag for this purpose: @remove NNC tag. Maybe you will find it useful.

Thanks. I didn't know about it.