Much, maybe even half, the value from my MA was the conversations with peers and colleagues during (and especialy after) our seminars together. It would certainly be more challenging to have these kinds of conversations in an online discussion - you'd be left learning much more independently, and while this is certainly possible, it might not be the same value as a full-time, in-person course of study with a good peer group.

On the other hand, if you'll be doing this degree while continuing to teach high school (which would require impressive time management skills to do the MA full-time, to be bluntly honest), you might not have time or opportunity to interact much with peers outside the seminars themselves. So the loss of interaction might not be as big a blow as if you were considering doing your MA without simultaneously working a full-time job.

You might consider doing an MA part time, as a full MA course of study is two or three seminars, often a language (in total. 9-12 hours of class time per week), in addition to which you'll be reading 2-4 academic monographs a week (depending on what your professors assign - but a minimum of 1 monograph per seminar per week), often plus academic articles, plus your research papers (one major one per seminar, plus book reviews and sometimes weekly writing requirements); ie a very significant time commitment unless you're a skilled speed reader.

I haven't done an online MA in History, but just looked over the program. I would totally do it if I weren't doing another online MS now. Mass is definitely a respected institution, even on the West coast. The biggest difference between this and other programs is that most of the MA programs in History I've seen have language requirements, and this one doesn't. As you're already teaching, that might not be a big deal for you. I don't think that lack decreases the value of the MA.