Hello rola.h.shewayhat,
I think Pisa and CT in one day is probably too much. My advice would be to choose between Pisa and Lucca as a day trip or concentrate on CT all by itself to maximize your time. La Spezia (the southernmost big train station near CT) is about 2 hours away by train - and almost as long driving directly - so you'll spend 4+ hours on transportation to CT all by itself.
It's a little over an hour from Florence to Pisa on the train. You can get into Pisa a little after 9:00 at the earliest and spend some time checking out the sites. Also keep in mind that logistics on the ground are a consideration. The Pisa train station is over a mile from tower site so either you'll need to factor in time for the walk or organize a number of cabs or public transit for this transfer both ways. Google maps public transportation tab has worked fine for me using buses in northern Italy. Buy tickets at the train station - probably a newstand or other store if you're taking the bus. (Bus tickets from Florence will very probably work there but I can't guarantee it.) Maybe the group is such that getting 12 people - including teenagers - across town isn't a concern but it sounds like a challenge to me.
La Spezia is another :45 to 1:15 away. I figure that puts you near CT about 1:00pm at the earliest and then a local train and there are lunch considerations given the number of people involved. See a town or two and then reverse this back to Florence. Pisa is on the way - you'll probably pass through or change trains in Pisa anyway - so I can see the appeal but adding hours to this already long day with that many people sounds like a lot to me, but that decision is up to you.
Cost considerations may or may not tip you back towards a tour to save time. The train is about 40e roundtrip per adult between Florence and La Spezia but times 6+ (kids 14 and under are 50%) the train tickets are going to set you back some money so there may be some advantage if you can organize a tour by van or something similar.
I can see how that many people makes planning such an issue since even cheap train tickets add up and there are probably too many people for a typical van tour. Hopefully someone with larger group organization experience has some ideas and recommendations but this is my take on it.
Hope you find a plan that works for everyone,
=Tod