You can't teach relative clauses in 30 minutes to a class of Junior High students. There is no point in teaching 'which' in islolation from 'who'.
You have to decide first of all which aspect(s) of relative clause you are going to teach. The grammars divide relative clauses into specifyng/non-specifying, or, alternatively into defining/non-defining. You can forget about that kind of terminology in a Junior High. There is a also a third category which the grammars don't mention. But in any case You can forget about that kind of terminology in a Junior High.
I suggest you bear two ponts in mind and then think of some illustrtive situations. situations
The first thing to bear in mind is that we use relative clauses following the noun phrase they refer to.
The second is that the kids will probably understand the diference between 'Which one?' and 'What kind of thing?'. This is the difference between specifying and non-specifying.
Try something like the following.
Teacher; 'Who do you love most?' 'Who is your favourite singer?'
Student: My mother
S. Because she is kind and understanding and looks after me.
T. Let's put these two things together. 'The person I love most is my mother, ....... .... .... ......................... .' (whois kind and understanding and looks after me).
This is obviously non-specifying because the student only has one other. The relative clause tells us what kind of person the students mother is. In writing we indicate this by putting a comma before 'who/which'
Specifying relatives can wait until next week.