Squibs?

[ladyramkin2000]

No: HPFGUIDX 92452

I've always blithely assumed that the people like the witch who 
brings round the trolley on the express or the barman in the Leakey 
Cauldron  are all squibs, since they are plainly part of the WW but 
are doing rather mundane jobs.  However, someone has suggested to me 
that they may actually be people who, for some reason or other, have 
failed their OWLS and have left school without qualifications.Is this 
possible?  Would they have been chosen in the first place if their 
magical potential is so weak?  And where does that leave people like 
Madam Rosmerta, who is strong and capable in her job and doesn't 
appear to need magic (or does she?)
Sylvia (who needs to think about something other than The Chat)