Is it fair that our troops with kids abroad must pay for schooling while the US pays $15,000/yr/kid to school work visa residents in US?

You are misinformed. Every developed country provides at least primary and secondary school education at taxpayer's expense for every child living in the school district. The only requirement is an address. Even in countries like Switzerland which has extremely strict immigration policies, parents are not asked to show that they are legal to put their children into school Feral children would cost taxpayers far more than educating them.

If the army allows you to bring your family, you can use the local schools. Mostly people do not because they do not want to bother to learn the language.

By the way, I have no idea where you get your $15,000 a year number from. My local tax payer funded school which is not even particularly good, costs $38,000 a year. There is one about 50 miles from here that costs $55,000 a year. That one is not particularly good either.

No, it is not. Neither is it fair to make taxpaying Americans pay for educating illegal-alien children. All those who are not normally resident in the US, but are instead are only temporarily present or not legally present (i.e. all illegal aliens, and all those on temporary visas such as student or employment), should have to pay out-of-district tuition for the education of their children.

Also note that Americans working abroad temporarily must pay for their children's education in most countries. That should go likewise for citizens of foreign countries working temporarily in the US.

i dont think anything in life is fair

No