in high school

[Unregistered]

I'm a Junior at Marie High School
I'm a Junior in Marie High School

I want to join the chess clue in college
I want to join the chess clue in college

[Soup]

These are correct. You needn't say/write the words in brackets (...):

[1] I'm a junior at Marie High School.
[2] I want to join the chess club (when I am) in college.
[3] I want to join the chess club at the college.
[4] I want to join the college chess club.