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(1) your spell points will be too low, requiring frequent resting.
Yes, but having level 2 or 3 spells is enough to rise insane high. Also you don't need resting. The trik is to have mana stones (3 or more the better). Each stone give arrund 200 mana with 3 charges. Go to He'li cast, use stone 2x then sell it-buy it. It will recharge the stone and you get virtually infinit mana. Don't bother with money. If you don't patch the game you have infinit money with PPing.
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low power of spells you can learn at that class level, so that the incremental gains from casting would be too low. Also, I believe the amount of practice required to raise skills goes up exponentially with the skill level.
Yes it is exponential. But not that extream. With a level 2 spell like charm you can raise up to 60 your school without much trouble. The realms goes up much faster. Tring to reach level 60 in the priest or psionic scholl with charm will bring your mental realm in the 90th. If you train with mindread you'll top the mental realm very quickly and your psionic school will be in the 90th without much pain.
The best is to constanlty cast the buffin spells with your spellcasters. That brings up the schools and realms very well. If you have hybrid calsses try to cast One spell at max power each fight. The fights will nearly always give training points to the eralms or schools.