I know nothing first hand about this. But I would guess that this is one more group of nuns who turned liberal post Vatican II. What was once a Catholic School is now a Montessori school, would seem to be an indicator.
I would also guess that if they are a liberal order, they are probably getting no new recruits into their order, and are all growing older. They need the money to provide for nursing homes and healthcare, the kind of job that younger nuns in the order would once have done.
It’s their school. They have a right to sell it. I suspect that the whole business is a sad one.
Maria Motessori was a very devout Catholic, but given that the school is run by lay teachers overseen by the sisters, I would bet that otherwise your post is spot on.
If education in the past has been subsidized by cutting corners on maintenance and retirement funds, and the sisters now have a need to wind down the order gracefully, nothing unjust is going on here.
Anyway, what's the problem?