I'm in a smaller town and we've had similar issues come up. It's been circulating in the back of my mind, so just to give you a stronger sense of what we saw more participation from -
https://betterworldfundraising.com/yankee-candle-fundraiser/ - this is the yankee candle one, and running it in the fall helped so people get items before the holidays because they tend to sell a lot of highly giftable items.
We found a greenhouse that let us sell flats of Spring flowers. Individuals from a school where it was normal to buy 1 thing would come in and spend $200 on a couple flats (and we might keep around 20%), plus a few hanging baskets and even planter pots that are pre-made. I'm in the North, groups also do sales for mum plants in the Fall if candles aren't exciting to the group. You would want to look up your growing seasons, and ask some places - some of our better deals came from places up to 30 miles away, so it doesn't have to be the shop down the street.
I'm pointing these out because while we had excellent participation in our elementary schools, I ended up the PTO president at our middle school... a school with about 800 kids and it was normal to have something like 20 of them even turn in their fundraiser forms in the previous years with the typical crap schools would send home. These ones would bring in 50 or so responses, and people were able to sell to other families, whereas the typical stuff we sent home was basically just parents buying from their own kids, which defeats this specific type of fundraising. We made a reasonable amount from the mug and bag sales fundraisers, but those were really kids selling to mom and dad only. The neighbor doesn't want an overpriced bag or school mug.
ETA- since these are all computer/science kids... I wonder if you could do a "hack a thon" or "code a thon" or something with the kids where they raise money by small donation- like when we run or jump rope or whatever... but something that is more fitting. Have the kids lock themselves in for a Saturday and code a roller coaster on roblox or something? They might be able to say what they have the skills to realistically pledge to do.