Grammatically, “We have to wear school uniforms” is the answer because “we” is plural and therefore you need multiple “uniforms”. However, in common spoken English you will hear “We have to wear a uniform” which is technically incorrect but is really a short way of saying “We each have to wear a uniform”. Either way it will be well understood. Another clearer example is “We have to buy a car” vs. “We each have to buy a car”. The first means only one car is bought collectively whereas the second means each person will buy a car.

The second one is incorrect because you you wear a uniform not uniforms. The first sentence is correct but you don't need to use the article "the" because you are talking in general about the necessity to wear a uniform. We know that it will be the uniform of the school you are going to... So: "We have to wear school uniform." Or, if you were talking to somebody who knew nothing about your school you might say: We have to wear a school uniform."

Has anybody heard the expression "a can of worms"? - or, as they say in the UK, "a tin of worms". 😉

It depends if the school has more than one uniform

Another point is the difference between “We have to wear school uniforms” vs. “We have to wear the school uniforms”. The first is used if you are already at that school and it is understood what the uniform being referred to is. The second would be used if you’re are deciding among schools and some require uniforms and some don’t. So there is no defined uniform yet so if asked about a particular school’s uniform policy, you’d say “We have to wear the school uniforms”.