Something lasting. What does the school need? Some schools are planting vegetable gardens to teach kids about healthy eating, so how about calling it the Teachers' Garden and putting up a plaque with their names on it? How about planting a nice shade tree (or several), perhaps a flowering variety, with a small sign - something to beautify the grounds? The town DPW will often do some of the work, and sometimes a local nursery will help by discounting or providing some of the materials. You could do a row of trees, to beautify the entrance area, each one with a sign for an individual teacher.
Perhaps a piece of playground equipment? Something for the computer lab or library? Something related to teaching and learning, that will benefit all of the students to whom these teachers have collectively donated so much of their lives? A lot of teachers also spend their own money on supplies that aren't covered by the budget - so perhaps a fund or a supply "bank" that everyone contributes to? There might be space considerations but it is a gift that would keep on giving. Or, set up a scholarship fund in each teacher's name that supports an area of importance to each individual. Some districts already have educational foundations, supported by fundraisers, that give out grants to individual teachers who apply for something special in their classrooms that is not covered by the budget.
In our area, a group created bracelets, with colored beads to match the school colors of many area districts. Two friends of mine work for school districts and volunteer to play piano for an area women's chorus that sings in nursing homes. They get a gift after every concert season, and one gift that totally touched them was a gift of a bracelet with beads in the colors of their respective town districts. Proceeds went to the arts programs in both schools. They were both incredibly touched by this gift, way more than by the very nice things they had gotten in other seasons, nice as those gifts were.
I think a memory book is nice for them to take home too - it takes a lot of work but if you have enough people, it can be done. If you have access to years of class pictures, it might be fun for someone to make a display for each teacher of all the kids they have taught over the years. If someone is clever enough, the display could be dismantled and turned into a book. You could ask for testimonials from current and former students, but that could become unwieldy unless you scan and turn it into a digital form.
Good luck!