Why doesn't INSERT AFTER Trigger add all results?

I'm working on the exercises for db-class.org where we have to write a trigger that makes new students named 'Friendly' automatically like everyone else in their grade. That is, after the trigger runs, we should have ('Friendly', A) in the Likes table for every other Highschooler A in the same grade as 'Friendly'.

This is being done in sqlite on the following dataset. Now my trigger works, but it only adds one person that 'Friendly' likes rather than all of them (those with the same grade).

Here's my trigger:

CREATE Trigger Q1
After Insert on Highschooler
For Each Row
When New.Name = 'Friendly'
Begin
    INSERT Into Likes Values (New.ID, (
    SELECT Highschooler.ID FROM Highschooler
    WHERE Highschooler.Grade = New.Grade));
End;

I suspect there's something wrong with my INSERT query, because in isolation the SELECT returns all the IDs. So how do I make sure it inserts all the IDs with matching grades?

On most platforms your insert would throw an error if your subselect returns more than one row, rather than just insert the 'first' returned row (and 'first' is undefined without an order by clause).

What you are probably after is an insert like this:

INSERT Into Likes 
SELECT New.ID, ID 
FROM Highschooler 
WHERE Grade = New.Grade and ID<>New.ID;

StackExchange Database Administrators Q#8045, answer score: 11