Music Theory is a site that contains lessons, exercises and tools that can be used in Music classes.
Learning and Teaching Resources
Ricci Adams' Musictheory.net is an American Music Teaching Resources website but has plenty of relevant Music activities and lesson plans plus utilities for music teaching. Very useful for key stage 2, 3 and 4.
The resources fall under 3 categories: Lessons, Trainers and Utilities which makes it very easy to find the exact resource you are looking for.
The teaching resources on this website will be extremely useful for most music teachers for students at key stage 2, 3 and 4. A good example is the simple but effective Note Trainer - one of many really useful student activities to bring music lessons to life.