Lets be more specific, you are not allowed to wear jeans/trainers/t-shirts Monday to Thursday, however every Friday is generally when the college "lets there hair down" and you can wear pretty much what you want.
Generally shirt and trousers is the norm (a tie is reserved for special occasions
)
This is a college policy and as you will find out, does not continue in other parts of the company.
Well done and good luck...