<human>: Hi all. I've been working in an elementary school for several years and have a very good relationship with my boss. It is a decent job, but before taxes come out, I'm only making about $20k per year (it's not full time). Additionally, I want to try something other than working with kids. During the summer, I was looking for full time work and had a few interviews but haven't received a good offer yet. 

Because I didn't want to be unemployed, I signed the contract with the elementary school and have attended staff training this week. I'll be teaching on a team of four, so it's not like I'll leave a classroom empty. However, I was interviewed for another position today, which seemed promising, and I have a second interview with another company next week. These positions are office manager and IT support, respectively. 

If given an offer, I would like to give my two weeks notice but I'm terrified that my boss will resent me for signing up to begin with. I'm trying to justify my departure by saying that I'm helping to set a good example for the new teachers. I'm still scared that this is a terrible thing to do, though. That said, other teachers have left mid-year as well. 

My questions:
1. If one company offers a job and wants me to make an immediate decision, what should I do?
2. How can I break the news to my boss if I get another offer? I'm an employee at will, so it's not so bad, right?
3. Have any of you been in a similar situation?

Can you condense the text into three sentences or less?
<bot>: Part-time educator looking to be a full-time office manager/IT person. Began the school year at elementary school, scared of burning bridges.