How to make a custom Drain Skill spell for training?

According to the UESP wiki, your method should work: https://en.uesp.net/wiki/Morrowind:Trainers

You can reduce training costs to 1gp per level by creating a spell or enchantment of Drain Skill of 100 for 2-3 seconds

If it's not, then is it possible:

1. You're running some engine modifications, like the Morrowind Code Patch

2. You're running some mods, like Morrowind Patch for Purists

Some mods or engine modifications likely affect this behavior, since it's deemed "cheesy".

However, you do have other options that may help and might be worth trying in the meantime:

1: Fortify Merchantile 100 points 2-3 seconds

2: Raise the trainer's disposition to 100, using any method you like