ANSWER: There are a several things to consider. While as you mention, marijuana is legal to people over 21 years old, the state is currently still developing the plan to handle the new legality of marijuana. As of right now, there are three categories of licenses: producer, processor and retailer. None of which applies to the situation you described. My guess is, however, that they will be developing laws to meet the demands of the new program and eventually we may see something similar to what we currently have in place for our activities that involve alcohol.
Also remember it is illegal to have possession of drugs within 1,000 feet of a school, school bus stop or in a public park, in a public housing project designated as a drug-free zone, in public transportation, and other locations and is punishable by double fines and imprisonment.
Even when the state has completed the process of creating laws to handle the legality of this question, the PTA would need to consider whether or not this seems like a wise path to follow. For many years, PTA has been vocal about the danger of drugs and have participated in both DARE and other anti-drug programs. This could send a mixed message to the children in your school community if you were to start accepting donations of marijuana for auctions and raffles.
In addition, such practical matters as where would it be stored, what would happen if there was a theft, and whether PTA insurance would cover it, must be considered. Finally, if your PTA does take possession of marijuana for an auction or raffle, you must ensure that in no way will it be accessible to someone under the age of 21 or you may be subject to prosecution.