1.Students aren't the only ones there
Yes, when you speak, your audience will be mostly students. But they aren't the only ones there. Teachers, principals and other staff members will know about your visit, giving your book an instant visibility. Depending on how involved the visit is, you might also get interest from the PTA or other parent board. All of these people could be part of your target audience.
2. Don't forget about the parents
Schools often send home flyers or info sheets to parents when someone is coming to speak. Again, this gets your book some visibility with people who might not have heard of it before. Half the game in finding readers is letting them know you exist. Encourage parents and students to work together to submit questions ahead of your visit that you can address in your talk.
3. Tie-in Events
While it's unlikely the school can open the doors to the community during regular hours, you might see if an evening event can be planned that welcomes parents and other community members to come hear from you. If the school isn't available, this is a great time to reach out to the local library. By hosting two events on the same day, you get more visibility with less travel.
Not everyone that reads YA is a teen. Likewise, not everyone reading adult genres is an adult. Just because your book isn't targeted to the age your speaking to doesn't mean those students won't be interested in reading it. Never underestimate a book hungry teen.
5. Bulk order discount
Since you're going to be there, you might as well offer the school a bulk order discount. This will encourage pre-sales and limit the number of books you have to haul back home. Readers love the chance to get a signed book on discount.
I suggest starting local with the schools closest to you so you aren't out much more than a half-tank of gas if the visit doesn't go as planned. You can always branch out later or include a more distant school visit as part of a non-marketing related trip.
If your book is too steamy or graphic for a middle school or high school, consider reaching out to colleges. They are more forgiving when it comes to content and professors are always looking for a way to link classroom lessons to real world applications.
So what do you think? If you write Adult, can you see a benefit in classroom visits? Any advice from those who have braved the rooms of 40 eyeballs all on you?