Each of the classes available to the player in Baldur's Gate 3 possess a variety of abilities and capabilities that allow them to explore the world and engage in combat. Each class can then choose a subclass, gaining abilities themed around that particular subclass that further diversify that character from others using that class. For the Wizard, the subclasses are known as "Schools" and one can be chosen for each of the eight schools of magic.
The Wizard in Baldur's Gate 3 can choose to specialize in any one of the eight schools of magic: Abjuration, Conjuration, Divination, Enchantment, Evocation, Necromancy, Illusion, and Transmutation. Each subclass choice offers the Wizard abilities relating to their mastery of a chosen school and bonuses when using spells that match their school. This allows players to choose which spell schools they want to get the most out of with that character and is especially useful when scribing spells into the spellbook from scrolls.
School of Evocation
The School of Evocation grants a Wizard the abilities "Evocation Savant" and "Sculpt Spells" at second level. The former halves the cost of scribing evocation spells into their spellbook from 50gp to 25gp per spell level, while the latter allows them to exclude allies from the harmful effects of their Evocation spells.
School of Abjuration
Wizards who choose the School of Abjuration get "Abjuration Savant" and "Arcane Ward" at second level. The former allows them to scribe Abjuration Spells into their spellbook for 25gp per spell level instead of 50gp per spell level, while the latter gives the Wizard a 3HP shield whenever they cast an Abjuration spell of level 1 or higher.
School of Conjuration
The Conjuration school is unavailable in the Early Access Build of the game, but presumably, they will receive "Conjuration Savant" and "Minor Conjuration" at second level, reducing their scribing costs and allowing them to conjure small non-magical items right into their hands.
School of Divination
The Divination School is unavailable in the Early Access Build of the game, but presumably, they will receive "Divination Savant" and "Portent" at second level, reducing their scribing costs and allowing them to roll two D20's after a long rest and then swap those results into a D20 roll that happens within range at any time between then and the next short rest (only once per turn, and only using each roll once).
School of Enchantment
The Enchantment School is unavailable in the Early Access Build of the game, but presumably, they will receive "Enchantment Savant" and "Hypnotic Gaze" at second level, reducing their scribing costs and allowing them to charm a creature if it fails a Wisdom Save against their Wizard Spell DC (Only once per creature, per long rest).
School of Necromancy
The School of Necromancy is unavailable in the Early Access Build of the game, but presumably, they will receive "Necromancy Savant" and "Grim Harvest," allowing them to halve their scribing costs and regain HP whenever they kill a creature with spells of spell level 1 or higher.
School of Transmutation
The School of Transmutation is unavailable in the Early Access Build of the game, but they will presumably receive "Transmutation Savant" and "Minor Alchemy," reducing their scribing costs and allowing them to transform common materials into other common materials.
School of Illusion
The School of Illusion is unavailable in the Early Access Build of the game, but presumably, they will receive "Illusion Savant" and "Improved Minor Illusion," allowing them to halve their scribing costs and get an extra cantrip for free (while also boosting its effectiveness.)
Baldur's Gate 3 launches on the 3rd of August 2023 for PC and Mac and on the 6th of September 2023 for PS5.