The rules on scrolls state:
A spell on a scroll can be used only once. The writing vanishes from the scroll when the spell is activated.
If she meets all three requirements but her own caster level is lower than the scroll spell's caster level, then she has to make a caster level check (DC = scroll's caster level + 1) to cast the spell successfully.
If the caster level check fails but without Mishap, is the spell "activated" or not? Is the scroll destroyed if you fail?