“Brainwashing” is a loaded term — it’s better to speak in terms of persuading people to adopt a highly specific worldview, and then evaluate that worldview to see if it’s problematic.
Some people criticize Bais Yaakov for giving their students the very strongly-held idea that marrying a full-time learner and being a kollel wife is the best thing in the world and if you don’t do that you’re a nobody, or at least on a much lower madreiga than you really should be. I don’t know if this is true — I don’t have personal experience with BY. But it does seem like a very limited and harmful worldview to impose on kids, since most will not live up to the standard, which itself is rather new in Jewish history.
If I were starting a high school, I would have the students learn a variety of frum hashkafic perspectives based on various classical and contemporary sources, so they don’t have an artifically narrow perspective, and can gravitate toward and further explore teachings that appeal to them personally.
In many cases, college (including the professors, course materials, and fellow students) really does persuade people to adopt very specific world views. In most colleges, especially liberal arts colleges but also larger universities, there is a very liberal, secular, anti-religious atmosphere. Deeply religious or conservative people tend to abandon their beliefs, and if they don’t, they are usually are too afraid to share them with anyone. It is much worse in the humanities and social sciences, compared to more technical or professional fields, but a few required electives in the liberal arts can be enough to transform a student’s worldview. I don’t think this is true at all colleges, and certainly not at places like YU or Touro, but it’s very widespread.