Adding NPCs- solving problems with lack of healers, etc.

Problem

I teach students with special needs in a pull-out class. The class is comprised of students with a variety of academic abilities, just like any other classroom. The main struggle I have with a majority of the class is the inability to plan for the future when we set up our initial teams. I have 2 healers in the class of 3 teams. With only 9 students in the class, there isn't enough healing to go around.

Solution

I am adding NPCs (non-player characters) to my game. Meet Winnie Swiftlight, Travelling Healer of Team "Travelling NPCs". Rigtht now she is the only NPC, but I could see myself adding a Mage and Warrior in the future.

Planning

• To get services from an NPC, someone on the team must pay by losing XP (The NPC does not gain the XP). I haven't yet settled on the exact XP cost yet, but I figure 20%-50% of what a student typically earns in a class period is fair. The more powerful the ability, the higher the cost.
• The NPC will gain XP by using the abilities. I will subtract XP as needed to prevent her from leveling up at the wrong time.
• The NPCs always stay 1 level below the lowest level player.
• I will give the NPC enough XP to level up to always stay 1 level behind the lowest level player character.
• I will mark the NPC absent everyday to prevent accidental XP gain when granting XP to the whole class.

Benefits

• This is all supposed to be for fun, so I'm seeking out opportunities for more. 
• No longer are students stuck at 1 HP after falling.
• If the healer is low on AP and multiple people need healing, perhaps an XP bidding war will ensue.
• I can subtly teach my class about economics.

Extension Ideas

• XP cost for services may be variable. I always love a reason to roll my oversized RPG dice. 10% of daily XP + 3d10.
• The cost goes up as the NPC's AP goes down. 80% of total AP gone = +80% XP cost. This will create a fair market price based on availability.