Why market to just teachers? Many teachers WANT summer care too..
If you are wanting to be a seasonal child care then I would go that route when advertising.
I don't know what the work is like where you live but here we have LOTS of workers that only work winter months and have summers off.
I also know there are LOTS of families that choose to hire a college student for the summer so their kids have more freedom. etc...
I would just advertise as a part year program in general and just make it clear upon enrollment that you close up shop for the summer months.
We have 3 daycares in my area that do it that way and they are always full and not always with teacher families.