@loveyouradvice - I have one going into year 13 and a Yr12 new starter. If you PM me I'll happily email you my packing list.
More generally I'd suggest the following -
fairy lights or bunting to jazz up their space. Both of mine have the type of lights that come with some mini pegs for clipping photos on.
Also, school is likely to supply a noticeboard of some sort in the dorm so perhaps a pretty sheet of wrapping paper to cover it and some pretty drawing/push pins to stick things on it. Last year DC1 was able to pop out to buy that but this year they can't go into town.
Both of mine have bought furry throws for snuggling, apparently v useful for chillaxing in the common room (if they are allowed in there with COVID rules) and a pretty cushion for their bed.
On the practical side a good supply of stationery if the school doesn't supply it, including the things like a stapler, paperclips etc that they might not remember for themselves.
A tube of travel handwashing detergent so they can handwash anything they don't want to give to matron (expensive lingerie bought with their birthday money tends to die in a communal laundry)
At least 2 heavy periods worth of san pro, multiple pocket packs of tissues, some nice mini bottles of handsanitiser, a pair of flip flops for wearing in the shower.
sewing kit plus spare name tapes and a laundry marker pen in case anything falls off/slips through the named item net and a shoe cleaning kit.
I suspect there will be a lot more time in dorms than last year so I would suggest putting the kindle app on their tablet/laptop and giving them some amazon credit so that they can instantly access a new reading book if needed.