I just had my two final interviews... what should I do next (Please read post)

I had two final interviews today and they went very well. One is for engineering firm (which I really want, top 500) and the other one is for a private school. The position is for administrative assistant. I’m a little stressed about what happens if I get offers from both of these companies. One(private school) is going to let me know mid next week and the other one still has to interview more people. Of course if the private school offers me the position I will say yes(I really need to leave my current job). What should I do or say to the private school after I said yes if I’m offered the other job for the engineering firm which I really want?

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