Why are McGonagall, Hagrid and Flitwick allowed to stay at Hogwarts after Voldemort took power

Question

Why indeed?

McGonagall and Hagrid are active members of the Order of the Phoenix. Flitwick is not but is obviously affiliated with them. Same goes for the other teachers - while they are not members, they surely sympathize with the cause and will take sides against Voldemort if they have to choose. Hagrid has to flee but just after he hosts the "Support Harry Potter" party.

It could be argued that Hogwarts is not of tactical importance (at least not short term) but keeping quite powerful wizards free is not quite wise.

In general most of the Order of the Phoenix seem to be ignored or just monitored rather than actively chased and arrested. The Weaslyes have to hide under the Fidelius charm but only after time.

So instead of eliminating the threat with a lightning strike for example by detaining the guests at the wedding they let them some time to regroup and go undercover.

Answer 1

Education of future wizards was extremely important to Voldemort. Mcgonagall and flitwick are to of probably the most accomplished wizards in their respected fields at the time. Voldemort knows that given the choice of fighting him, or teaching children, the children are their number 1 priorities. Also while replacing them was potentially possible, their was still attempts to keep everything stable, and appear to not rock the boat, everything he did was in the shadows, and through manipulation. Parents who saw a complete change of staff, after Dumbledores death, would undoubtedly panic far more then it appears the average family was.

Answer 2

Many of the above could be seen as correct.

Voldemort would arguably know that McGonagal and Flitwick were part of the Order, just as much as he'd know Hagrid was. What he wouldn't know is which of the students were. The Professors could be seen as trophies but also as symbols that there is no resistance.

He'd be less concerned with the education or the parents than actually controlling these young, impetious wizards. Despite his mocking of Neville, there is some admiration there for his "pluck" and he'd be a valuable ally if he could be turned for example. Prior to the battle, the best way to keep the students docile and compliant is to present as close to "business as usual" as possible rather than cracking down by killing the old guard in front of them. He is bargaining when he is asking about Harry rather than outright threatening, he is telling them "your lives won't be much different except you now serve me, but if you don't your lives will be over." The threat is always present but he rarely overtly threatens people who he thinks might have value to him, he offers them the chance to join of their own free will...

He knows those 2 teachers and particularly McGonagal would never risk a students safety for herself whatever the personal cost to her own moral and likely hopes she too will eventually join him willingly and at minimum will do as told without the need for the Imperius curse at pain of killing off kids as punishment. He'd never kill her, just make her watch him kill her Gryffindors one by one.

Hagrid is useful from the practical point of view. He performs functions at Hogwarts that need performing whoever is in charge, again he won't risk the kids safety by being surly or rebelling outright is Voldemort's thinking, he knows Hagrid's weaknesses and can play on them easily.

Answer 3

Because McGonagall and Flitwick were perhaps the most powerful two teachers in the school along with Snape. He would not want them roaming free able to communicate with the other Order members or have the chance to assemble a battalion to oppose him. He was probably frightened of losing them as teachers and as they were pure or half blood he would want to keep them. He may have had personal fear as the two fantastic forces put together may have been able to overthrow him personally. Why he kept Hagrid is beyond me.