I have no idea whether they are good with SEN - AFAIK no-one in DSs class had any issues like that. In general, they do teach the whole class at once - though the max I think is 15 to a class, so there is a better ratio. But still, they can't really teach in sets, even when they reach the exam classes. So, I don't know how the teachers differentiate the work if you're not going at the "class speed". I'd certainly say you should speak to them directly about that, to get an understanding of how they'd arrange the work if she is struggling.
There's certainly less distraction - the teacher can see if they are chatting amongst themselves, as it all appears on the screen (like instant messaging), and can cut off a pupil if they are causing disruption to the class, even taking them to "another room" to tell them off while the class carry on together for a short time.
We were certainly happy in our dealings with them - certainly had no more issues than we did with ordinary schools.
I think one of the first things you need to think about is what you hope to get out of HE, and why you don't find school effective. Interhigh is very like school - that has pluses and minuses. So, you're learning in a group, and have specialist teachers, which suited DS well.
OTOH you have no flexibility about the classes or subjects, you are in an "unsetted" class, you have no flexibility about the timetable for the week or the year.
So, a lot of people who choose to HE because school doesn't suit thier DC would probably find that Interhigh didn't suit them either. But if your problem isn't about the way school does the academic stuff, then it might work well for you.
As I say, it was just a practical school choice for us, to let DS get a lot of music time, rather than him not gtting on well in a school environment.