Spent about a year as a day pupil (i.e. didn't actually board) at a boarding school and have a large number of close friends and acquaintances who spent their entire school lives at them.
For context these are the stereotypical English posh boarding schools... think the Eton
Are these boarding schools more strict than a "normal" school? Almost certainly yes. They are in many ways the last open remnants of Empire and the class system (although that's been whittled down by the arrival of "new" money) and they pride themselves on installing discipline and the stereotypical English "stiff upper lip". School uniforms are mandatory throughout the school day, adults in general (and teachers especially) are referred to by title (normally "Sir/Miss") and rules and regulations are observed more strictly then you'll generally find in a "normal" school. That doesn't necessarily make them "oppressive" but it's a step along the way.
Then you have the boarding element itself. If you're living at home your parents will often cut you some slack from time to time... either because they're your parents or simply because for whatever reason they're not around/that deeply involved. In contrast at a boarding school it's literally the job of matrons, house-masters and the like to keep you in line and while they may come to care for you deeply (it's worth remembering that if a kid spends their whole educational life in a boarding school that the staff there will almost certainly spend more time with them then their parents do) that doesn't change the fact that it's their job
. A parent who normally imposes an 11pm curfew may make an allowance or simply not be in a position to check... someone at a boarding school who does the same risks losing their career. It leads to an almost benevolently antagonistic relationship between staff and pupils not entirely dissimilar to a more light-hearted take on inmates and guards in a prison; pupils keep attempting to get that bit more freedom and the staff keep trying to catch them doing it.
The other oppressive issue with boarding schools is the bullying. Unlike at a "normal" school where if one is getting bullied one can at least go home at the end of the day and at weekends at boarding school there's basically no escape; your bullies live where you live. Combine that with the stiff upper lip concept mentioned above and the abuse can be horrifying. There have also been structures that largely codified that abuse; "fagging" was a system where junior pupils were expected to serve the more senior ones and while the name originally had no link to sexual activity there was a huge amount of sexual abuse. It's largely died out (at least in the semi-formal sense) but there very nature of boarding makes it a risk.
I can't really comment on the "straightening up"/reforming aspect of these schools because that wasn't really what they were about; parents send their kids there for the prestige and the education rather than to sort out trouble makers and most of the kids start attending at an age where they haven't even had a chance to rebel yet. What I will say is that if anything those schools tended to do the opposite in my experience. Do you know the stereotype about catholic schoolgirls; went to a strict convent school and then as soon as they left started drinking, doing drugs and sleeping with anything that moved? In my experience that applied to almost all the boarders I know (male and female). With things like alcohol and sex being such taboos and the nature of boarding making it so difficult to engage in them once they got the opportunity they drowned themselves in it... which was helped by the fact almost all came from staggeringly rich families so they had the money to burn. To use myself as an example, while I had my mid-to-late teens to get used to drinking, a bit of drug taking and sex so by the time I was at university and had a lot more freedom I was "over" them to the extent they weren't a massive deal (not that I didn't still enjoy them...) the boarders I knew had never had the chance to normalize those things.
There's also been some psychological research that suggests boarding from a young age can be harmful in the long term; the effect of your parents in effect "abandoning" you can damage the psyche, especially combined with some of the issues mentioned above.