Rat out
Definition 1 of 1
Pro Tip 1/3Informal Speech
Used in casual conversation, often among peers or within social groups. 
If you rat me out for cutting class, you're not cool.
Pro Tip 2/3Negative Connotation
Carries a negative sense, suggesting disloyalty or breaking trust. 
I can't believe she'd rat out her own team!
Pro Tip 3/3Non-professional Context
Usually inappropriate in formal or professional settings, where 'report' or 'disclose' might be better. 
In the office, it's better to report issues than to rat out co-workers.