School of exuming

Exuming corpses is a easy process the entire goal of which is to increase the numbers of your horde of undeath.

Master of the dead

Beginning when you select this school at 2nd level, the gold and time you must spend to copy a necromancy spell into your spellbook is halved.

Exumer of many

Beginning at 2nd level, For every undead you would normally summon or make you instead summon a number of them equal to half your int modifier rounded up (minimum of one)

Exempt from burial

Starting at 6th level, When a target you can see dies you can spend your reaction and transform them into a zombie.

Mob mentality

Beginning at 10th level, If a zombie you control has 3 zombies within 5ft of them than their ac and attack and damage rolls will gain a +2 bonus

Horde caller

Starting at 14th level, You can call fourth your army of undeath with a wave of your hands. Once per long rest, you can use an action to summon a group of zombies who's number of zombies equals your (int modifier x your proficiency bonus) + your wizard level