。
I spotted a boy running away from the classroom.
Literal
I [topic-は] boy [subject-が] classroom [from-から] flee-going [nominalizer-の] [object-を] spotted.
見つける here means 'to spot, catch sight of.' The nominalized clause 少年が教室から逃げていくの describes the scene observed. ~ていく expresses movement away from the speaker's vantage point — the boy was fleeing in a direction receding from view.