If i want to make tokens for say university grading system (ex. each grade for every student in a course is a token he got ) , what's the best ERC token standard should i follow ?
You dont need to follow any token standard; just need to write a smart contract that containing student's grade in smart contract's storage.
Every grade is unique, even if the value of the grade repeats itself. Say a 90 grade for math today, and a 90 grade for math in past period are different. This means you have to go for ERC 721 standard, non-fungible token.
However, your idea of using tokens for grades is conceptually flawed, since grade (as a concept) should not be transferable because it becomes an attribute of a person (sort of). In Ethereum you can transfer tokens from one address to another and grades should not be transferable because it would look like a fraud.
You don't need tokens for such simple data storing as it's not tradeable. Just create
Struct and that should be fine.