Edit: She's used an iPad since Grade 2 (she's in an IB program so equivalent of NZ Years 3) which has been great. The school requires laptops from grade 6.
If the school doesn't care then whatever fits the budget. What do most of the other students use? If your wife is on staff then she should know. Everything will be web based so OS won't make a difference. If she has the same OS as most then it will make it easier on the teacher.
We’re in the Middle East. It’s a mixed bag what other kids use as the school allows a variety of laptops. Some have MacBooks, some use windows.