How come anyone with let's say a death secondary school gets stronger death cards than a death student?
You don't, at all, in any school. In fact, you get no bonuses from your secondary school, such as the blades/traps, etc. What you might be seeing are treasure cards - those are usually "better" than the school spell.
I'm guessing they are using a death mastery amulet with treasure cards. With that combo they can use whatever death spell they wish provided they have the necessary pips. Letting people use higher level spells that should be reserved for primary schools only, is one of the problems made even worse with pip mastery from amulets.
It can also be from what you're wearing. You can buy and craft clothing that can give you bonuses for your secondary schools (you can buy crafting guides when you get to Krokotopia). I have a +12% strength to all my Death spells because I crafted that hat and a pair of boots from the guides I got in Krok (and I'm Storm). Also enchantments and treasure cards.
