When choosing between used to and would, they often mean the same thing. When that's the case, my advice is to go with the more direct option. In other words:
When I was a child, we would go to the beach.
is better than
When I was a child, we used to go to the beach.
For one thing, the word "used" can trip up the reader, because we use it in different ways:
We used plastic pails to build sand castles. We used to go to the beach.
So, in your case, I would use would, not used to.
As far as differentiating between would draw and drew, that depends. If you are referring to a particular event, then say:
When I was in junior high, I had a classmate who drew scorpions on paper, then showed it to students to scare them off.
But if this was something your friend did regularly, you might say:
When I was in junior high, I had one classmate who would draw scorpions on paper, and then show it to students to scare them off.
(I don't think you need a second would, because the first would can apply to both verbs. Saying:
When I was in junior high, I had one classmate who would draw scorpions on paper, and then would show it to students to scare them off.
sounds a bit verbose.)
As a side note, you might want to be careful with the word "off." Sure, scare off has a slightly different meaning than scare, and show off has a slightly different meaning than show. But, using two offs so close together sounds off, and makes the reader wonder if either was necessary in the first place. So:
When I was in junior high, I had a classmate who drew scorpions on a paper and showed it off to students to scare them off.
becomes:
When I was in junior high, I had a classmate who drew scorpions on paper, then showed it to students to scare them.
(A good way to improve your writing is to remove unnecessary words, then reread your sentences. If you haven't lost any meaning from the edit, then you should probably leave the words omitted.)
P.S. Your junior high classmate sounds like one twisted individual.
