It's not too complicated to organize this. Just email Jimmy at the Arboretum to make sure that the forest classroom is available on the day you plan on taking your group, and not reserved for a visiting school or an activity run by the Arboretum.
Explore the trail beforehand just so you know the general direction - no worries, you will still get lost the first time you take a group of 40 - I sure did, what makes you think you'd master it right the first time, huh? :P Warn them ahead of time - a little self-deprecating humour works wonders!
Watch the weather forecast closely. Good thing is you always have your regular classroom as a backup. Tell them to bring sunscreen, hat, and appropriate clothing and footwear (no heels, it is a forest trail, not a paved walkway).
In case you are wondering, Germany was playing in the FIFA Cup that day ... :)
What else can I say? Oh yes, bringing treats will earn you lots of brownie points, too! :) I raided the German store on my way to school.
And the only catch besides the weather is that if the winds are blowing from the south, all airplanes headed for Pearson will approach the runway by flying above the Arboretum.
So don't plan a lecture session!
Besides that, I can highly recommend it, especially when you are teaching on a hot summer day!