... at the moment I'm in the midst of reading Susan Brown's Orphan, which is very good by the way, and was reminded of another similar story.

I can't remember it's era or if it is set in the UK or US, here is what I can (may be not in sequence order)

• Protagonist is lower class/street rat type
• Protagonist is only child of single parent
• Protagonist's parent dies
• Protagonist is harassed, by overzealous truant/CC officer
• Protagonist hides in the attic of boarding school
• Protagonist realises boarding school is a girls school
• Protagonist saves life of overzealous truant/CC officer
• Protagonist is discovered by a pupil at the school

Pupil is daughter/niece of a highly placed/powerful family (they have chauffeurs, butlers and house staff) Protagonist is invited 'home' with pupil.

I also can't remember if this particular story is complete or not but, either way it is a good story, but if I'm right and not mixing multiple plots the protagonist is adopted by that family.