Briarwood: Schools used to get more money from the city when they employed senior teachers because schools were funded based on employees' average salaries. Then Mayor Bloomberg changed the policy and senior teachers became a financial liability. No senior teacher, no matter how talented, would be hired by a principal who had a choice between one expensive veteran and two cheap rookies. This is the real reason many excessed teachers can't find jobs, though principals are more than happy to let them sub, clearly trusting them to teach.
And notice how the savvy Ms. Silverstein even knows how members of the Absent Teacher Reserve pass their time: they teach!