Sometime at about age 11 inexpensive Sheaffer cartridge only fountain pens seemed to appear in every school supply section of every store.
It became "cool" in my school to write with a fountain pen again. The Sheaffer school pens were not great writers, as I recall, but they worked and from my perspective, you didn't need a bottle of ink, which would have been a non-starter with the teachers.
It was exactly these cheap Sheaffer's, which stores stocked for twenty years, that I looked for when I got curious about fountain pens again. Unfortunately, they seem to have disappeared.
At first it seemed like the stores where we shopped had a selection of colored ink cartridges, but over the next couple of years this seemed to get narrowed to blue or ugly blue/black. I still cannot stand blue/black ink.
You can still find these pens on eBay for a couple of bucks. Frankly, I see no reason to buy on used, other than nostalgia. They were nice for their day, but you can get a Chinese made fountain pen, new for maybe a dollar more.
My use of these pens lasted until early high school. In high school it just became easier to carry a couple of ball points around since they never leaked or dripped.