As a teacher, I’m not really sold on the Montessori method. I think it works best for preschool children and possibly Kindergarten. It does not work as well for school-age children because in practice, the concepts they absorb through play or self-directed independent activities with manipulatives aren’t always explained to them later. It’s like showing a kid a pyramid and expecting them to notice, fully understand, and remember that all the sides are equal without ever telling them. This is why the true Montessori method is rarely used as children move up through grades - they have to incorporate traditional teaching methods or the children won’t learn the large amounts of information they need as they progress. When properly used, however, I think the method can be effective for certain children in the younger grades.
There’s nothing wrong with donating money to a Montessori school, though! There are children there who need funding for school supplies and activities just like at any other school.