Parent is right, a mix of platforms is inevitable. Might as well take advantage of the opportunities, instead of trying to preserve a monoculture. The school board could quite reasonably insist that all digital school work must be submitted in open file formats. They could acquire a wide range of hardware, continuously, and allocate higher or lower spec machines depending on the actual needs of each class or grade year.

What the school loses in simplicity, they gain in budget flexibility. What the student loses in familiarity, they gain in resiliency. Change and diversity cannot be stopped or squashed. Open source does take a bit of work, but it pays for itself, partly in lowered risk, especially for large non-profit or low-budget organizations.