My advice:
1. Keep quiet about the other job with the school
2. Finish your instructor rating
3. Take the job thats been offered by the other company
If, in the meantime, the school offers you a job, say you're considering it. Still complete the CFI first then make a decision. I think you'd be crazy not to take a punt at the job that has been offered.
Loyalty is not that common early in the industry and companies realise that guys are always looking to advance in the industry. That being said, "don't crap where you eat". You never want to burn bridges...better to leave on good terms.
Hope this helps.
HP