[Radek]

1. You cannot get Stone Magic as an Assassin - at least, not in my game. Assassins get Fiend and Moon schools but not Stone. The only sources of Stone Magic are Mages and Valkyries. Note: I am playing without any patch.
2. You are not right, Ryanamur. That's you who is losing mana points. Perhaps, I didn't express myself well. Therefore, I meant the following:
a) Create a mage. Put him in the inn.
b) Bring him ankhs so that he can change roles.
c) Bring him a skull and a spider egg. Make him a warlock. Return him into the inn without leaving the town.
d) Bring him a raptor egg. Make him a ranger. Note that the adept of a ranger has to bring the egg - he need not kill the raptor. Therefore, the forerunner takes the ranger quest, kills the raptor, brings the eggs (there are 2 eggs available), gives one egg to to the mage and the second one to a Chest sitting in the inn and cancels the ranger training. Now, the mage takes the ranger quest and finishes it immediately. Return the mage into the inn without leaving the town.
e) Dangerous action! Let the mage take the paladin quest. Bring the mage to the Gael Serran Altar - run like hell there and back using the raft or - if your forerunner has a Moon magic - use Create Portal/Teleport/Call of Home. It may happen that you'll need to fight. In this case, do not level up your mage. Note: the ordering of role changes is selected so that the mage is now a ranger and can be protected by a chain mail at this moment.
f) Let the mage take take the barbarian quest. Do not level up until you become a barbarian. Note: the ordering of role changes is selected so that the mage is now a paladin and can use katanas and plate mail during the barbarian quest.
Why all these tricks? Because we want the mage to have all magic schools AND we want him to have as few apells as possible and as low skill values in magic schools as possible when he becomes a barbarian. Now, the babarian gets mana points in each magic school when he levels up. The amount of mana points gained equals to the barbarian's SPI so that we also want the barbarian to have SPI 24. The process of gaining mana points stops at some moment - the lower your magic school skills are and the few spells you have the later the process stops. Note: you can improve your Sorcery skill without spoiling the process.
An example: if you have SPI 24 and if you raise all magic schools to 1 (so that you see all magic schools in you skill list and can improve them during leveling up later in the game), then the process stops after 15 increments. You will have 380 SP in each magic school. Now, if you add 10 spells to each magic school (by means of leveling up! not by means of using books of magic!), you get additional 240 SP in each magic school. Result: 620 SP in each magic school.
If you allow gaining spells or raising some magic school skill, then the process of gaining mana points stops almost immediately. Well, if you keep a particular magic school undeveloped completely then you'll see an increment sometimes (rarely). Trainig Stone to 7 and Sun to 3 equals losing the advantage completely. You are losing mana points.
g) Become a warlock after getting your 380 SP in each magic school. With SPI 24, you will gain next 24 mana points with each spell added to your spellbook during leveling up. After filling your spellbook completely, you will have 700 - 800 SP in each magic school.