While "back to schools" might sound grammatically correct, it's not idiomatic. The phrase "back to school" refers to the general concept of returning to education, not necessarily to multiple schools.
Here are some examples:
* "The kids are getting ready to go back to school."
* "It's time for me to go back to school and finish my degree."
You can use "schools" in other contexts, for example:
* "There are many different schools in the city."
* "The government is investing in new schools."