This is probably better addressed to a California lawyer because it may require specialized local-area knowledge. But since the California lawyers are strangely silent I'll try to help.
If this was a federal case (this is before my time—I can imagine there having been state laws against violating the Selective Service Act in 1968), I suggest that as a first step you contact the district clerk in the federal district where the charge was filed (the Central District of California, (213) 894-...
Criminal mischief can be a misdemeanor or a felony, depending on the amount of the damage (more than $1500 worth is a felony; less is a misdemeanor).
Of course your son can dispute the charge. Criminal mischief requires intentional or knowing (not accidental) damage.
See my answer to your previous question. A class-C deferred is not legally a "deferred adjudication" (though we sometimes call it that as shorthand for "Suspension of Sentence and Deferral of Final Disposition"). So nondisclosure is not available for a class C deferred.
Nothing is definite, but if she doesn't sign the ticket they might get a warrant for her. They probably won't go looking for her, but warrants tend to pop up at the least convenient moments—she's driving somewhere important, gets pulled over for speeding, and winds up in jail.
The age of consent in Texas is 17; that is, a 17-year-old can legally consent to having sex with anyone, even a 44-year-old. So it is (barring any other facts) legal, and you have no duty to call the police.
As Rosario and Paul note, since you pled guilty or no contest to a felony, you're stuck with a felony on your record.
My colleague Rosario's solution—wait five years after the probation is terminated and get an order of nondisclosure—will not help you much, since state licensing agencies (including whoever licenses psycologists) are one of the groups that will still have access to the records even after nondisclosure.
I have to disagree with Paul on one thing: a deferred adjudication does not require a no-contest plea.
180-days-probated-for-one-year sounds like a straight probation, which can't be expunged or sealed.
One small correction to Paul's answer: as of 9/1/2007 there is no statute of limitations on sexual assault of a child, so if the government finds out that you and your boyfriend had sex when you were 16, he could be charged with a felony at any time.
John Brewer has been prosecuting in Harris County for a long time—more than 15 years. Many criminal defense lawyers in Houston will have dealt with him. Search avvo for "criminal defense" in Houston, and you will find a number of excellent lawyers. Call and talk to them, and make appointments with the ones who seem to best fit your needs. Find the lawyer you trust, and find a way to hire him (or her).