MONTANA LAWMAKERS in 2015 enacted a tax-credit scholarship program premised on the principle that every family — regardless of income — should be able to choose the school that best fits the needs of their child. Hoping to take advantage of the program, Kendra Espinoza, a single mother, took her two daughters out of public school, where one had struggled academically and the other had been bullied, and enrolled them in a private school. The girls thrived at the Stillwater Christian School, but paying tuition was a constant touch-and-go, with Ms. Espinoza forced to work three jobs and even hold yard sales to sell off belongings. Now, the girls’ ability to continue at the school is wrapped up in a U.S. Supreme Court case with implications for school-choice programs nationwide.