The crossing guard is not a police officer and the same standards of tolerance likely do not apply.
Here is the law (assuming this is what she was charged with):
http://www.statutes.legis.state.tx.u.../htm/PE.42.htm
It would appear she is guilty of 42.01(a)(2)
Sec. 42.01. DISORDERLY CONDUCT. (a) A person commits an offense if he intentionally or knowingly:
(2) makes an offensive gesture or display in a public place, and the gesture or display tends to incite an immediate breach of the peace;
Whether the gesture alone is sufficient to meet the legal requirements that it "tends" to incite a breach of the peace or not, I can't say. But, it certainly was not intended as a gesture of "it's okay" or "peace and love."
If this is an offense that can carry jail time, she should have the right to have an attorney assigned to her by the court if she cannot afford one. However, if it is such that it is a fine only, appointed counsel may not be an option.