The word "may" suggests a possibility, not an obligation. Does "free clothes" mean no uniform?
Student or Learner
I am in a class. If a teacher says "You may wear free clothes tomorrow", would that mean that you have the choice to wear free clothes whether you want to or not or does it mean you must wear free clothes?
What about 'can'?
"Can" does not mean you must.
"Must" means you are obligated.
I hope you understand from Mike's reply that "free clothes" is not a phrase we use.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
Os something like You are free to wear what you want tomorrow.