Non-genetic, environmental, post-infectious, or psychosocial factors—while not causing Tourette's—can influence its severity.  Autoimmune processes may affect tic onset and exacerbation in some cases. In 1998, a team at the US National Institute of Mental Health proposed a hypothesis based on observation of 50 children that both obsessive–compulsive disorder (OCD) and tic disorders may arise in a subset of children as a result of a poststreptococcal autoimmune process.  Children who meet five diagnostic criteria are classified, according to the hypothesis, as having Pediatric Autoimmune Neuropsychiatric Disorders Associated with Streptococcal infections ( PANDAS ).  This contentious hypothesis is the focus of clinical and laboratory research, but remains unproven.   
As a middle school teacher myself, I’m not at all surprised by this letter, and not really annoyed by the tone, either. This age group is notorious for blowing things out of proportion, and a letter reminding the parents to keep an even keel and remember to pay attention to their kids in light of this incident seems very normal to me. The tone is more a function of our litigious society – they have to cover every base, because there are many people who would like to find an excuse to sue anyone for their kid’s emotional challenges. That’s the unfortunate part in my mind.