Do you think it's ok to wear university clothes even if I didn't attend that school?
That depends on your reason for wearing the clothes. Why are you?
I'm wearing a school shirt to make a person think that I went that university.
In that case, you shouldn't trick people into thinking you went to a school you didn't go to. If they find out, they will not trust you in the future.
I don't care.
It's best just to be honest with people instead of lying to try to impress them. They almost certainly don't care about if you went to a certain school or not. Lying about where you went will just cause distrust in your friendship.