Do unions really control school boards?

The folks at the Society for Quality Education posted two items Thursday and Friday, cumulatively questioning whether union endorsement and donations to the campaigns of trustee candidates means those unions end up controlling school boards. From the Friday piece:

Because the average voter is hard-pressed to be well informed about all of the electoral races and issues - mayor, local councillor, regional councillor, referenda, school board trustees - many teachers gratefully accept their union’s recommendations and vote the party line. While lots of people mark their ballot only for mayor and maybe councillor, leaving the school trustee part at the bottom of the ballot blank, most teachers do get all the way down the page and vote for the trustees too. And of course, many retired educators run for office, increasing the chances that their former colleagues will make the effort to vote for them: this is reflected in the fact that a higher percentage of teachers vote in elections than most other occupations. All in all, it’s safe to say that education providers are well represented at the polls.

SQE beleives it's safe to say that these endorsements lead to trustees' successful runs for office being on the backs of the teachers whose very unions endorsed them. It's an interesting extension of logic, but I'm not there yet. Do I believe more teachers vote than other occupations? Probably. Do I believe they all vote for the union-endorsed candidate(s)? I'd hope teachers are better at critically evaluating candidates, regardless of any endorsements. Given those doubts, I wouldn't attempt to say teachers elect trustees who are friendliest to their own interests.
If I extend what I understand to be SQE's logic, does that mean there are council candidates out there in municipalities who owe their elections to police and firefighter associations? Both types of unions also have a very long history of endorsing candidates and making campaign donations to those candidates friendliest to their interests. Given councillors and mayors provide direction for firefighter bargaining (the little that doesn't seem to be settled by private-sector arbitration) and also sit on police service boards that set police budgets, shouldn't we be as outraged over that as SQE is trying to make us over teachers' union endorsements?
Further, so much of the collective agreements (dollars, scales, broader working conditions) are now being negotiated at provincial discussion tables that this only dilutes any one trustee's impact on one particular contract. SQE points out that trustees sit on the management side of the table at negotiations. While technically true, I'm not aware of boards where trustees attend these sessions-- rather they provide direction to their director of education, who then bargains based on that direction.
Despite diverging on opinion (again) with SQE, I would note it's once again using publicly available data to make a point. If you agree or not with their concern, you should be aware that all municipal / school board election candidates must by law declare their expenses and any donations over $100 (I believe, it's been awhile since I looked at the limit) to the municipal clerk. The clerk must then make these results public. I believe the timing is around the end of March or some time in April following a municipal election, so if your interest has been peaked, drop by your township/town/city hall and take a peek.