Check this website to get you started on research. Specifically under Watch, Read, Listen, two items will be helpful. The top item under Watch, Read, Listen is Rick's Travel Articles, scroll down to Italy and you will see some specific articles. Also under Watch, Read, Listen, pay attention to Audio Tours at the end of that list. I used Rick's audio tours all over Italy and found them to be extraordinarily good--and free! They need to be downloaded onto people's phones here in the U.S. though because they take too long to download there.
I would suggest stopping in Florence for a night or two also, since it is distinctively different from Venice and Rome. Some students may feel more comfortable in a smaller environment, and the market there is ideal for exploring different foods and letting them be high schoolers. This might substitute for a food tour, which could be pricey.
There are "free walking tours," tips expected, in all three of those cities. Check out whether one might be suitable for your group.
With students this age, involve them in the planning. They can do some of this research, watch youtube, etc., and then if they are involved in the planning, they are more bought in, will act up less, and will be more prepared and interested. Get them an Italy guidebook, Rick Steves is excellent, and have them start to discuss options.
My last piece of advice is to begin to prep them for riding the trains, which I think are by far the best transportation method within Italy. Between those 3 cities, they are quite efficient too. If you plan to do this, buy the tickets far in advance for the best price. Suggest the seat61.com website for seeing what the trains are like. This will lead naturally to the discussion of them packing light so that getting on and off the train is easy. That's enough from me. Others will chime in.