Asking for letters of recommendation for HS teaching positions
If the TA supervisor knows that you are leaving the program to find a (high school) teaching job, and can speak to your ability or potential as a teacher then their letter will be a plus.
As for feeling you cheated the program - better to leave now than after taking their support for a few more years and leaving then. Some fraction of students leave without a degree - it's expected and not shameful.
Finally, I suspect high school physics teachers are in demand.