Early in Act II, Mary tries to extort a bribe from Rosalie in exchange for knowledge of what is happening at the school. Mary asks, "Will you give me a free T.L. if I tell you?" What is a "T.L."?
"T. L." is an old abbreviation (originating in the 17th century) that is meant to signify the intent that something is meant to remain a secret. The "T" is the initial for the Latin word "tace," to "be silent." The "L" represents the Latin origin. So, "T. L.," or "Tace is Latin," is a way of conveying a message that is meant to be a secret. Check out the link below for a more historical description.http://www.enotes.com/childrens-hour/q-and-a/what-does-t-l-stand-for-used-by-mary-actii-when-7775
The above answer is helpful for telling you what the TL means. By specifically saying "free" TL, Mary seems to be asking for a favor of a future secret. When it comes to girls and gossip, trading secrets is important!