Once she blow my whistle, she know it's dismissal

from A$AP Rocky (Ft. ASAP Ferg & Gunplay) – Ghetto Symphony Lyrics on Rap Genius

Meaning

Once a girl gives Gunplay head, he has no more use for her.

He needs to write a book called “Gunplay’s Guide to Women.” Tell me you wouldn’t read that!

Also obviously wordplay based on school — once the whistle is blown, school is dismissed.

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