I am a parent in this school.
First of all, it is an incredible school. Far from what has been assumed here and wherever this video is being discussed, they give over an excitement and pride of Yiddishkeit to their students. The principal is an incredible woman. She knows every girl in the school, and any of my daughters will tell you that she loves them. When people talk about how schools should be teaching students to love and live Yiddishkeit instead of just seeing it as a list of dry rules, they should be looking at this school for how to do that.
All that said, there is nothing wrong with a school having rules and enforcing them. The school has always had a rule that when hair is worn down it should not be beyond a certain length. This year they extended that, saying that all hair should not be longer than 4 inches below the shoulder. This rule may be new for this year, but is very much in consonance with the overall rules of the school and standard of tznius that they have had for many years. Any parent who is upset about this rule didn't do even basic research about the school before sending there. This is not an out of town community school. This is Lakewood, with countless girls schools, and whoever sent here did so because they were looking for this particular standard.
Incidentally, the school did send out letters to parents signed by a prominent local Rav who is on the school's Rabbinical advisory board, asking mothers to be aware of how long sheitels have become standard and suggesting a sheitel length. Mothers are not students, but the school did ask parents to be partners with them in maintaining a standard of tznius.
For those who are calling this child abuse, just don't tell my daughters they are being abused. Nobody actually there thought it was a huge deal, more like it was funny. None of my daughters are confused about what is halacha and what is chumra, none of them think long hair is the same as not covering knees. They know the halachos of tznius and they know that there is an overall tzura of tznius that the school is aiming for.
Finally, as others have mentioned above, this was during nit check. It was not during school. There are no more than a small handful of people in the room at any time, and nobody was made to feel embarrassed. The girls whose hair was too long were simply told to please get a haircut before school started. There was no shaming, no public spectacle... and it was done by a secretary, not even a principal or teacher. Again, not one girl in the room looked like it was a big deal. Yes, it was new, and girls who liked their hair long may have been upset to have to cut it, but none of them felt embarrassed or "abused".