Once you've trained through a second school (using your training points), then you can train at a third school. But, keep in mind, you WILL need training points.
ok then Miss Nat i will be learning my 3rd school soon because the last card i have left in my secondary school which is fire and that cards name is helephant so thanks to all who posted :-)
The best Ive found is to use your 2 schools completely and with the remaining 8 training points in the fairgrounds of wizard city get the anti-sheilds for each school. I am a master pyromancer with storm as a secondary although I mostly use just fire). With my anti shealds and using my primary schools I find I can do aproxamatelt 17,000 damage before standing the chance of being defeated because i have a good defence.
:-)