Transitional kindergarten is now available to every four-year-old in California, it is taught by credentialed teachers in the public system, and it costs families nothing. Any private school that tells you it is not a serious option is selling rather than advising.
When the free place is the right answer
If your child is four, TK covers the hours you actually need, and you would move them into public kindergarten the following year anyway — take the free place. Buying a single middle year of a three-year Montessori cycle is the weakest version of the Montessori case, and we would rather say so than take the enrollment.
Where the comparison usually goes wrong
Families compare a tuition figure against zero. The fairer comparison is the whole arrangement you would need: a TK day does not cover a working day, so before- and after-school care goes on top. Once it does, the gap narrows — sometimes considerably.
Where the Montessori case is strongest
Not in one year, but in the shape of several. The three-year cycle pays off in its third year, when a child becomes the oldest in the room and moves from concrete materials into abstraction. And then, at Mills specifically, elementary continues in the same method to fifth grade rather than handing you back at five.
The test we would actually apply
Go and watch both. A TK class and a Montessori morning, on ordinary days, and notice what the children do when nobody is directing them. Twenty minutes of that is worth more than any comparison table, including ours.