Both are correct. There is only a slight difference in emphasis. #1 is a report of an historical event: the school closed two weeks ago. #2 is a report about what's happening at the school right now: it's closed right now and has been closed for two weeks.

They’re both correct but are used in different contexts. The first might be used in an official document or statement relating to the school in a more general context : ‘It is two weeks since the school closed, and the traffic congestion caused by the delivery and collection of children has ceased to be a problem.’ The second is more likely to be used in discussing the consequences of the closure and its implications for the immediate future : ‘It has been two weeks since the school closed and there is no evidence of vandalism of the premises.’ Use of the present perfect suggests that the situation is still ‘in play’, and that there could be further consequences.

It has been two weeks since the school closed.

Technically, they are both correct. However, the first sentence sounds awkward, at best!