Hello everyone, and thanks in advance for helping me solve this relatively minor issue!
Here is the story-
I have been "subbing" for a fellow teacher friend (Miss Theresa) all summer at a year-round education center. Theresa will happily return in two weeks, but I would like to be an on-call sub during the school year. Also, I would like to be considered for the same position next year, if there is an opening.
The job is two days a week. I work T/Th and another "Little Learner's" teacher (Miss Penelope) works M/W. No program on Fridays. It is only for three hours.

Penelope has been trained for working with bigger kids, and will go back to her after school programs/tutoring when school begins. She likes the "Little Learner's" program, but she is better with bigger kids.
I have subbed on Wednesday three times this summer for Miss Penelope.
I know Theresa would call me during the school year if she needs a sub, but I may not be able to because my "real" job is as an almost full-time sub at the private school my children attend.
Not sure if Theresa will teach all four mornings, or if they are hiring another teacher to split two mornings each, or if Penelope will continue with the little kids, splitting two and two with Theresa.
Anyway, since this was a sub job understood to be summer only, do I still turn in a two weeks' notice? How do I word it? Also, how do I say, "please call me (preferably a day before) if you need a sub." ?
How about this:
Dear Boss Man,
Please accept this as my formal resignation as teacher for the Little Learner's program, effective (date.) I greatly appreciate all I have learned here and would be delighted to be an on-call substitute during the school year.
Also, please consider me if you need a teacher next summer for the same program.
Thank-you for allowing me the opportunity to be a part of your educational center.
Best regards,
Region Mom