I'm not living there anymore so info could be well out of date but I did grow up in the system. If it's like it used to be the schools aren't full.
Kendal:
Kirkbie Kendal School - smaller of the town's two comps. Decent school. Good sports facilities due to being next door to leisure centre. Average results.
Queen Katherine School - bigger than KK and, despite being the old secondary modern to KK's old grammar school status, now generally the better regarded and better performing of the two. Very strong in music I think.
Windermere:
The Lakes School - not good, I don't think. Was in special measures a while ago.
Milnthorpe:
Dallam School - quite small I think. Reasonable results. Hot on MFL (eg used to offer Russian)
Kirkbie Lonsdale:
Queen Elizabeth - popular choice I believe. Has Performing Arts Status. My mum was blown away by a show there a while back.
Coniston:
John Ruskin - very small for a secondary. Not sure it has the best reputation.
Sedbergh:
Sedbergh - only goes up to 16 and small but a decent school I think.
Penrith:
Queen Elizabeth - not a clue about this one.
Keswick:
Keswick School - I don't know it personally but I heard lots of things about selection by the back door and how everyone wants to go there.
To be honest, I don't think you're going to get a really badly performing school in Cumbria until you get towards the far North West of the county (some of the Carlisle and Whitehaven area is pretty grim) and I don't have a clue about any of those, I grew up in South Lakes.