Rather than using existing classes from the script editor, it might be better to write your own example classes that are tailored to what it is you're trying to teach. If you do this in a script directly above Main, and use the global space to create instances of the example class to work with, you'll be able to use the start up period to demonstrate what is going on underneath, inside the code.
A script that demonstrates calling methods, no classes or objects involved, could look like this:
def method_one
#When calling method_one, print out some text
p "Test from method one"
end
def method_two
#When calling method_two, print out some text
p "Test from method two"
end
def method_three
##When calling method_three, print out some text
p "Test from method three"
#Method_three also calls method_two and method_one
method_two
method_one
end
#We want to call each of the three methods above in turn
method_one
method_two
method_three
I threw in comments because it's easier to understand what is happening, after it's been written. But as you are also saying whilst showing, you can explain it as you go. No classes, no modules, no nothing except methods. Then you'd go on to showing arguments and parameter passing, and explaining how those work.
I'm not sure what the scope of your tutorials are, but if it's Ruby in general, it'd be best to stay away from the existing scripts and stick to using your own. If you also want to integrate that into RGSS3 and the default scripts, then you can work that in slowly. But by starting from a basic empty script it allows you to introduce different aspects and topics over time, rather than doing all this stuff at the beginning where you are copy/pasting text around (which is a habit you shouldn't teach, because that can lead to problems if you aren't too confidant in scripting/programming).
You can also avoid bringing in things you haven't talked about, like modules and classes, which to a first time user might be raising more questions than you are answering.
If you want something to look at for ideas, you could check out Gubid's tutorials on youtube. He starts off with a few introductory videos, but mostly uses a project based approach, like demonstrating how to write a particular scene like a cooking/crafting script.