You will be confronted with some challenges, though: Moaning bus drivers, finding a good drop off point for, guests, who have troubles walking, queus in front of the biggest sight. In this article I am going to summarize how to deal with these.
Moaning bus drivers: Not long time ago you could still drop off you passengers in front of the Scala (guests can‘t go inside this opera house! You need a ticket for an evening event). It is at the Piazza della Scala, named after the opera house, which had been named after the church Santa Maria della Scala, built in 1381 here. Now there definitely are cameras when you enter that zone, in which coaches without permission are not allowed to go to. If you plan some days ahead and the coach company is willing to organize the entrance for you: Fine. If not: Go to the castle, ideally the main entrance.
Those, who know Milan would immediately know what you are talking about, when you refer to Sforza Castle (Italian: Castello Sforzesco) simply as the castle. It is very beautiful to see and from there you basically go straight ahead to reach the Duomo. Actually the official name is Duomo di Santa Maria Nascente, it is a catholic church and was dedicated to the holy mary. If the driver can‘t stay, because all the parking spaces are full: Make a simple trop off and tell him to be back at a certain time, so the guests don’t hear it. Tell the guests to be there 5, or 10 minutes (depending on punctuality of your guests) EARLIER to make sure, the driver does not need to wait, which he probably can‘t.
Now, having left the bus, they want to take pictures of the castle. It is big, so 10 minutes won‘t be enough. There are public toilets in there (follow the signs). From the main entrance of the Castello you basically go straight ahead to find the Galleria, which is beside the Duomo. The Galeria Vittorio Emanuele II, named after the first king of the kingdom of Italy, had been built between 1865 and 1877 and is famous as the prototype for the modern glazed shopping malls. (Wikipedia). That is the city center and more is really not possible within a few hours. So the walk takes between 10 and 30 minutes, or even more, if you have people with walking problems. For those, I suggest the underground. It is only one, or two stations and very quick. You find it right behind the big fountain, which is in front of the main entrance of the Castello.
Please tell your people already on the vehicle, that they shall not plan to enter the Dome of Milan or go up the tower of the Duomo (even though it really offers a great view). First you need to queue for the ticket, than at the entrance. Each takes an hour, if you are lucky. (A more exclusive travel agency might have made a reservation to avoid long lines.) The Duomo is already fascinating outside, especially when you look at all the figures, which had been carved with so many details. If they intend to go to the city center and are hungry: Luini, Via Santa Radegonda 11 (2 minutes from the Galeria and Duomo), for a typical snack from Milan. Luini is also the name of the typical dish you should try. Other than that: When you see the Duomo, go until you almost touch it and than turn left to go into the famous Galeria. Go through the Galeria and right when you exit it, you find the Scala, the famous opera house. If you want affordable coffee: Walk a few minutes away from the center. If you don‘t care about money, try the nice terrace Aperol. It is right to the left of the dome, when you look at it, next to the main entrance of the Galeria. You have to watch out, it is not very obvious, as you need to go to the first floor.
Your passengers might ask you why you don‘t go here, or there. The answer simply is, that the city is too big to go to all these places within such a short time and for getting an overview the city center is the best option. Secondly traffic is so crazy that circling around is pretty much impossible.