There are 25% and 30% versions of Hex TC. Being that they stack with any school I believe they decided not to make it more powerful. Yes Curse is 20% and 25% TC, but Hex is craftable. Hex Item cards come in all 25%, 30% and 35%, mostly 35%. Also notice that feint doesn't increase in damage percent either.
The OP is correct, in almost every other instance the TC version of a spell is stronger, regardless of whether it's school-only or not. Hex and Feint (as seethe42 stated) are a couple of the few exceptions, and to my knowledge there has never been a logical or empirical explanation as to why this is so. It looks completely arbitrary.
Balanceblade (Balance only) is 25% boost, while TC Balanceblade is 30% boost, and Eye of Bartleby TC Balanceblade is 35% boost.
Stormblade (Storm only) is 30% boost, while TC Stormblade is 35% boost
Storm Trap (not Storm only) is 25% boost, while TC Storm Trap is 30% boost, and Eye of Bartleby TC Storm Trap is 35% boost.
Death Trap (not Death only) is 30% boost, while TC Death Trap is 35% boost, and Eye of Bartleby TC Death Trap is 40% boost.