I can really see the Dilema here...

Get the interveiw scheduled on a school day and knock off... take it and if they agree to hire you leave the other job... if they pick some one else your old job is still safe...

I really can't see the problem you're having here...

What exactly is tough about it? you scared of having to be assertive and say 'I quit' if you get the better job or something? Scared of either bunking off school or asking for the lesson off? Scared of asking the clothes shop if they can change the interveiw to an hour that suits you?

What's the problem?