Sure. But if you take X% of the students out of a school, the school does not become X% cheaper to operate; there are fixed costs and economies of scale.

If a charter (any charter) takes 400+ students out of the Somerville district (which is their eventual goal), the best-case scenario is that either a school closes and some students have to go further to get to school, or our property tax rates go up. It's not without costs.