Parker 45

They can take a while to flush out (like a Parker 45), but they work very well. The grips also seem to be much much more durable than the similar looking slimmer Montblancs of the 1960s (Montblanc 12, 14, etc) which are prone to hairline barrel cracks.

Vintage pens. The Parker 45 and Parker 51 are designed for long writing sessions. Not just in their weight and shape, but in the reliability of the ink flow over extended use. (I get so tired of having to prime the feed every 15 minutes in some of my modern pens.).

Best bang for buck if you don't mind some wear. Probably best to find one that has been refreshed, but that should still come under budget unless you go for a particularly rare example. Pen_Swap is always a good place to find some, though dangerous to the wallet lol. The Parker 45 for example is a popular one and has a 14k nib.

However, it gives you the best bang for your buck in its price range, with the (possible) exceptions of Pilot Capless and the Platinum 3776. Regarding vintage pens: last weekend I paid about 5 euro for a Parker 45 Flighter Deluxe with a 18 k gold nib, in really good condition.

The m80 is a remake of one of my favorite pens, the Parker 45. Check eBay for Chinese pens, a lot of vendors offer free shipping. If you’re in the UK you might be able to find a vintage Parker 45 in your budget even... they are very nice pens, use modern Parker ink cartridges/converters, and have a really convenient slip-on cap (great for jotting a quick note).

Also every once in a while places like endless pens will sell them off at $12 of so like how I got a black and pink one with a medium and broad nib at that price. But if you mainly want it because it's said to be great for school there's a few choices that are regularly under 20, such as a Pilot Metropolitan (aka MR), or you find a used vintage Parker 45 (a hell of a workhorse, very reliable, and takes modern Parker cartridges or converters) which a lot of times shows up under 15 to 20 and just needs a good flushing of water thru the back to be usable again.

Vintage Parker 45: used "base model" ones can be had off eBay for $20. Bit of a pig in a poke with a used pen but they are dead simple pens that will last forever. Can be had NOS and there are a wide variety of nibs to be had on the cheap and gold nibs can be found occasionally. Usually found in fine.

I'm happy to see it won't go to waste. Never had a Parker 45, but my hubs said they're wicked reliable right out of the box. A solid pen. He mentioned if it's one with a gold nib it could be worth a little something.