Hi, Northern Beaches Secondary College has a number of campuses - Mackellar Girls, Balgowlah Boys, Manly Selective, Cromer and Freshwater. Cromer is comprehensive, Freshwater is a senior campus (similar to a 6th form college I believe). My daughter goes to Mackellar and is very happy there. There's also Killarney Heights, Davidson and Forest High. Relatively inexpensive private schools would include Oxford Falls Grammar, Stella Maris and St Lukes.
All the best.