Since Earth Day falls on the same day as the FFA State Convention (April 22), some advisors have used it as a substitute teacher activity in the past. Or you could start the project mid week and have students finish it while the others are at the convention....
There's a paperback called "The Green Book" that is a useful resource for the project, or there are many resources on line as well.
Joe C. Green
Horticulture Teacher - Pope High
FFA Chapter Advisor
I.S.A. Certified Arborist International Society of Arboriculture
770-578-7900, ext. 286