It is grammatically correct to say both of these sentences. The one with "must" means that the school insists (i.e. you have no choice; obligation). The other one, with "have to", can be used to mean exactly the same thing, but if you want to suggest that you think it is important to wear a uniform, use "have to" - and in speech, put the stress on "have", not "wear".
|link comment||edited Oct 27 '11 at 06:54 Agreeonpurpose Contributor|
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