What should I do?

[JollyNot]

I have been offered a job, which I accepted on Weds. Today I have been invited for an interview for another position, similar to the first but with an older age group, different school. I'd prefer the older age group, the school is much nearer and I feel they offer more in terms of career progression. So, I'd like to attend the interview.
However, I obviously am not guaranteed I will get the second post. So what do I do? Wait until I know and then decide? Be upfront with first school and explain reasons why? Put off signing a contract for first school until I know?
I don't want to mess them around but I do want to feel like I am giving myself the best chance.

I've thought I might just tell the first school I'm away for the next two weeks to delay....

I've thought I might just tell the first school I'm away for the next two weeks to delay

Hmm that is messing them around though, isn't it? Is their window of opportunity during the school holidays? If so, then you delaying their process by 2 weeks, possibly longer, while you attend another interview, then it gives them a recruitment problem.

If you had posted that you hadn't accepted the job, then I'd be saying go along to School 2 and then decide. The fact you've accepted makes me feel you need to make a decision, which probably means coming clean. Hedging your bets isn't very trustworthy if you've accepted. But that's only a personal opinion, of course.