three positive things about being in school teaching.
1) christmas,
2) easter
3) summer.
that is all...
You forgot:
1) Autumn half term
2) Spring half term
3) Summer half term
Mickey, the only person who can decide what to do is you.
First and foremost, what do you want out of life? Do you want a challenge and a career that is in demand? If you do, then go for teaching. Working in a shop is, well, working in a shop. Nothing wrong with that, but what are the job prospects? What's the highest salary (realistically) that you can hope to get? Will you still be happy working there in 5/ 10/ 20 years time?
Now compare that with working as a Teaching Assistant. How do they compare?
Hellcat also makes a good point in that by going the TA route you can try the job without making a huge commitment to University training. If you like it, then great, you could always go on day release and get fully qualified. If you don't like it, then nothing is lost. Believe me, deputy manager jobs of a shop are easy to come by so you'd have no trouble in finding another job. Chances are as well that your old company would want you back anyway.
Another thought is to maybe take the deputy manager job for a year and use it as a kind of 'gap year' experience. It would look good on your CV/ application and shows that you have the means to take on responsibility, plus the extra cash would come in handy!
Lastly, I think you are still in your teens so to be honest don't fret too much. I went back into full time education at the age of 36, and started my degree course at 37. It's never too late to re-train, BUT don't leave it too late! I think I'm pushing the boundaries a bit as I'll be 41 when I fully qualify - I could retire in nearly a couple of decades!
Whatever you do, follow your heart and take the career that you really want to do. As someone once said, "If you do a career that you love then you'll never have to work again".