The police can not "interrogate" a minor without the parent in most jurisdictions. The principal is not a law enforcement agent, and he is free to question a child at his school. If you feel that your child is being bullied, obtain the procedures of your school district for having it addressed. In most jurisdictions, the school is obligated to investigate, but if their investigation shows that it is not actionable, or did not occur, then they are not obligated to act. If you fear that your child's retraction will hurt your case, then you can bring up the fact that your daughter felt intimidated by the principal and that's why she did that. You have other remedies outside the school as well. You can have a local lawyer send a letter to this girl's parents, because they are in fact responsible for what she does because she is a minor, if you can show they know about a particular tendency i.e., violence or they should have known--once the lawyer sends it in writing, they've been legally put on notice. If she threatens your daughter, call the cops. The lawyer can also tell you if you have a good case to get a restraining order (schools tend to be afraid of that because if it is granted, they have to monitor and keep them 100 ft apart in most states).
No, an attorney cannot do anything for you at this point. If you do not like what the principal did, you can file a grievance as provided in the Board of Trustee Policies. You can find a link to them on the Plano ISD web site, under the page for the Board of Trustees. Follow it exactly as written to preserve your rights.
Regarding the bullying, you may want to look at the materials at the Texas School Safety Center (http://txssc.txstate.edu/topics/bullying/) including the bullying flow chart.
This is general information and should not be construed as legal advice. Without knowing the specifics of your situation and engaging in an attorney-client relationship I am unable to provide specific direction.