Toy hash algorithm that can be broken I'm looking for a toy hash function, where the idea is to have high school students break (i.e. find a collision) a hash function by hand, in order to teach them how one way functions and hashing works.

Do you happen to know some function like that?

As for how hard I want it to be to breakable: Ideally a "sizeable" function so that depending on some variable it can be easier or harder, but overall I'm looking at something about between 10 and 30 minutes.
I like the didactic approach in this answer. But for something that behaves more like a hash function should, we might define $H$ for strings of digits $s$ as: $H(s)=(((1||s)\bmod 97)||s)\bmod 99991$ where $a||s$ is the number resulting from prepending the decimal representation of $a$ to the string $s$.

Worked out example for $H(012345678)$
$=(((1||012345678)\bmod 97)||012345678)\bmod 99991$
$=((1012345678\bmod 97)||012345678)\bmod 99991$
$=(37||012345678)\bmod 99991$
$=37012345678\bmod 99991$
$=77082$.

With some math insight and very little extra computational effort, one can exhibit a collision; a "brute force" search for collision is also feasible with simple use of a spreadsheet.