It means there is a deeper meaning. In your example, it's nice not to wear a uniform but there are other good reasons too, less obvious to see.

"there is much more to it than that" This can be literal, but often is used to indicate that there is additional or secret meaning. ---- For example, "I thought that getting good grades in school would get me a job, but there is much more to it than that." <- This is a literal type of usage where it might indicate that in order to get a good job, I need to do things besides getting good grades, the things could be "family connection", "having experience". For example: "I thought that getting good grades in school would get me a job, but there is much more to it than that such as knowing the right person who get help me get an interview." ===== Another example, "I think that they will break up their relationship because they are always fighting over money, but there is much more to it than that". ====== It is NOT usually used to indicate more work or effort is needed - this would be abnormal: "I ate the first two courses of the meal, but there is much more to it than that"

This means 'That's not the whole story' or 'The situation is much more complicated/important than that.' The speaker would then go on to explain what he or she means. In this case, they'd probably explain why the change in clothing rules is so significant to them.