No, there are no rules/laws that restrict how much you keep in your accounts. Non-profits are a type of business. Depending on how huge they are, some have millions in their accounts. (Probably not PTO's
) Small or large, a non-profit needs to operate within its mission and ensure that individuals (i.e. organizers, not clients) don't personally benefit in an inappropriate way.
You do need to check your bylaws to see if there are any rules within your group about account balances. Beyond that you need to find out if there are any school or district policies that might affect you.
Finally, and most importantly, you want to use common sense for your group. Most groups want to spend the bulk of their earnings within the school year to benefit those kids that raised the funds. Some raise funds this year to fund the following year. It's fine to save funds for a big project like a playground that takes multiple years to raise. But it's generally a bad idea to keep a lot money just for the sake of keeping it.
Note, most groups do maintain a reasonable balance to kickoff the following year and support starting activities. (My group keeps a balance of $2000-$3000.)
Good luck with your situation!