3-level Hierarchical SQL Query

I'm very well aware of basic SQL queries, however I've never worked with a query that deals with hierarchical data.

I'm using the Telerik TreeView control to display data related to a school board with the following structure:

--School "ABC"
----Class "Grade 1"
----Class "Grade 2"
------Student #1
------Student #2
--School "DEF"
----Class "Grade 1"
------Student #3
----Class "Grade 3"

The TreeView is in a tri-state checkbox mode, so each element can be checked off. So if a single child element is checked then the parent element will be checked as well, and when checking a parent element all the child elements will be checked.

After checking off some schools, classes, and students, the following screen displays information about the students in a graph which currently uses the school IDs (if multiple are checked) to select all students of those schools.

Here's where it gets tricky. Using the above structure as an example, I select the class "Grade 1" from the school "ABC" and class "Grade 3" from the school "DEF" which in turn will select students # 1 & #2.

As mentioned before, my current SQL query is based solely on the school ID and I know that I can't simply add two other conditions in the where clause that look like this:

AND ClassID IN ('Grade 1', 'Grade 3') --Note there is no primary key for classes, and I can't change that in my position..
AND StudentID IN (1,2)

as this will also select student #3 from the other class title "Grade 1"

So my question is, how to I combine the School ID(s), Class name(s), and student ID(s) into one condition that will solve this issue.

Any help would be greatly appreciated as I'm really stuck on this question with a very strict deadline, thanks!!