The obvious answer would be that they are not taking government subsidies so the first amendment rules. If one is taking government subsidies, then the government has the right and obligation to set the standards for which to continue receiving it. Do they not do that to states?
Do you not remember the Welfare Reform Act signed into law by Clinton that did essentially the same thing? That is until the Democrats changed all of that with new law that opened the flood gates to abuse and corruption.
Then you are admitting the goal of the effort is not to stop truancy, but to punish the parents of truants who happen to be welfare recipients.