While schools rugby is lagging way behind in terms of coverage in comparison to Ireland and South Africa here in England. It is developing, quite quickly.
You have quite a few websites covering it very well such as:
http://www.inattheside.com/
http://www.fifteenrugby.com/
While you don't get the 30,000 people derby games often do get quite good crowds. A notable one would be the John Fisher School vs Whitgift school; the Croyden derby which attracts around 2,000-4,000, which is considered a HUGE amount this a typical local state school(john fisher) vs the posh public school lads (whitgift). My local derby Wimbledon College vs Kings College, Wimbledon is also very much like that except it gets around 500-1,500.
There is a national cup running called The Natwest Cup which has been running for 20 years or so(?). The current champions are Dulwich College who have now won it 3 years in a row. Link:
http://en.wikipedia.org/wiki/Daily_Mail_Cup
This is also covered by the national newspaper "The Telegraph"
http://www.telegraph.co.uk/sport/rugbyunion/schools-cup/
The finals day of this (u18 main Cup, u18 vase, u15 cup, u15 vase) usually attracts around 10,000-15,000. The semi-finals and finals are also streamed online on the above websites listed and the RFU website.
So like i said while schools rugby isn't very big at the moment it is certainly growing and the Rfu to their credit are also putting in a fair share of cash into it.
And although it is difficult to compare as schools usually only play schools in their own regions (unless they go reasonably far in the national cup) the main rugby schools in England would be:
Wellington College:
http://www.schoolsrugby.co.uk/Team.aspx?ID=46&TID=191423
Dulwich College:
http://www.schoolsrugby.co.uk/Team.aspx?ID=99&TID=194283
John Fisher School:
http://www.schoolsrugby.co.uk/Team.aspx?ID=888&TID=194337
Whitgift School:
http://www.schoolsrugby.co.uk/Team.aspx?ID=265&TID=194378
Warwick School:
http://www.schoolsrugby.co.uk/Team.aspx?ID=258&TID=194485
Sedbergh School:
http://www.schoolsrugby.co.uk/Team.aspx?ID=236&TID=194749
Millfield School:
http://www.schoolsrugby.co.uk/Team.aspx?ID=38&TID=192120
Woodhouse Grove School:
http://www.schoolsrugby.co.uk/Team.aspx?ID=269&TID=192557
RGS High Wycombe:
http://www.schoolsrugby.co.uk/Team.aspx?ID=278&TID=191397
Now im not saying that they are the only big schools as they are definitely not. Just some of the most notable. I also have a bias to London/South East as that is where i live.
Sidenote: Only one of them schools (John Fisher) is not a fee paying (private) school.
Hopefully this post gave you a pretty good insight into English schools rugby, excuse my English i am writing this in a rush and not checking my grammar.