a. K-5 to 2nd grade - Too young. I wouldn't trust them to clean a bong let alone use a lighter to fire one up.
b. 3rd grade to 5th grade - Still too young. They don't have the dexterity to roll a good joint at this age and it would just frustrate them to try and teach them. Besides, they're still able to go flying around torqued up on sugar most of the time, so introducing them to speed is just a waste of time and resources.
c. K-5 to 5th grade - see above