Sizing for school trousers - is it just me?

[noarguments]

Is it impossible to get school trousers to fit properly or are my kids just a strange shape? We've been backwards and forwards (school uniform shop, John Lewis, M&S etc etc), and the ones that approximately fit round the waist are ridiculously too long, so much so that to adjust the hem would make them look ridiculous.
Any tips?

God, the sizes are so weird. Dd3 is a tall almost 12 year old. She has a pair of m&s casual trousers, age 14, that fit her beautifully. So I tried buying m&s age 14 school trousers - massive on the waist, too short in the leg. She has Matalan and BHS (Tammy girl) skinny (i.e. style wise, not "slim fit") age 16 now, which fit nicely. Not sure what to do when she gets too tall for them!

Next go up to age 16 with elastic in the waist and M&S go up to age 16 then up to waist 24 with elastic in the waist. You have to order the bigger M&S sizes.