We have a school district public library board considering requiring background checks for new employees. They are concerned that they may be legally required to background check all current employees. Would there be any legal reason they would need to do so?
[NOTE: for background to this short answer, please see the much longer "Ask the Lawyer" https://www.wnylrc.org/ask-the-lawyer/raqs/205, that addresses the tightrope walk/legal minefields of employee background checks.]
So, does a school district public library implementing a background check for new employees have to also check their current ones?
The answer is: no; barring an over-ruling requirement (such as a term in a union contract) a library board can implement a background check policy for all hires going forward, without imposing a "retroactive check" requirement for current employees.
However, I would never advise that approach. Here are three reasons why:
1. Possible discrimination
A policy to only check the backgrounds of "new" employees could have a disproportionate impact on candidates on the basis of age, or gender, or race (to name a few). By not checking everyone, an employer risks the appearance of (or actual occurrence of) illegal discrimination.
2. Possible liability
Employee background check policies are implemented to reduce risk. If an employer is using employee background checks to reduce risk, there should be a very good reason for not checking all employees (such as a union contract that bars it), or the employer risks a claim of negligence.
3. Worker relations
A work environment should be a place of high trust. By subjecting one class of employees ("new" employees) to heightened scrutiny, in addition to the possible concern mentioned above in "1," it creates an unbalanced environment for trust. This is bad for morale.
I appreciate that background checks can come with a cost, so minimizing their frequency is helpful. I encourage any library implementing such a policy to check with their "Directors & Officers Insurance" carrier, since sometimes, carriers offer resources to defray and even pick up the costs of the check.
Thank you for a thoughtful question.
 Of course, if a school district public library is in a school (not a common scenario; school district public libraries are largely autonomous and separate from school district property), and if the librarians are on the payroll of the district, then they are already being background checked and fingerprinted, per the chart here: http://www.nysed.gov/educator-integrity/who-must-be-fingerprinted-charts. Of course, this question pre-supposes that the board is setting the hiring policy, which means the library is autonomous.
 Just to be clear, a contractual obligation to not conduct criminal background checks should never be in a collective bargaining agreement! However, some reasonable restrictions on the scope of such a check would be consistent with NY law and policy.