Mark's response below is great from a criminal perspective. My perspective is from one who mostly handles employment law disputes.
This conduct is inappropriate. You should review your employer's sexual harassment policy and make an internal complaint pursuant to that procedure. Ideally, a confidential investigation will ensue, and the employer will take prompt remedial measures. If they fail to investigate the claim or take such measures, they find themselves strictly liable for the harassment.
Note that harassment, in order to be prohibited under Title VII of the Civil Rights Act or the Pennsylvania Human Relations Act must be 'sever and pervasive.' I also trust you have let him know in no uncertain terms that these efforts are unwelcome. I would also preserve the emails.
If you need further guidance, or if the conduct continues, it would be a good idea for you to seek the advice of an employment lawyer.
If a fellow employee is a lovelorn suitor -- even a persistent one -- it may not necessarily be stalking or sexual harassment, especially if you're able to deal with it appropriately. Sometimes, an over-zealous suitor needs to be counseled by an appropriate supervisor without turning his efforts into a legal affair.
However, if this fellow employee follows you without permission or other appropriate business and places you in reasonable fear of injury or emotional distress, he is committing the crime of stalking, in violation of the Pennsylvania Crimes Code, 18 Pa.C.S.A. Section 2709.1. If he sends you emails frequently enough or takes any other steps that annoy you or alarm you, he may be guilty of the crime of harassment, under 18 Pa. C.S.A. 2709(a). This cover repeated unwelcome communications, lewd or obscene language, and following you in public places. All of these criminal matters can be reported to police, if you believe they are serious enough -- and criminal charges will then issue against this employee.
You did not indicate whether you work for a public or a private school. As an employee, when a co-worker makes sexual innuendos, you may have a viable claim for sexual harassment (which is treated in the civil court system as a form of sex discrimination). You may have rights to file a claim with the EEOC or a court of law under Title VII of the Civil Rights Act, as well as other applicable law. The law protects against "hostile work environments" -- an expression that is specifically recognized under Title VII and refers to an uncomfortable sexually-charged environment. I would report this situation immediately to a supervisor. If you ever chose to pursue a sexual harassment claim down the road, your failure to make a timely report of the events can be used against you.
If you were related to your stalker, or had a prior intimate relationship with him, you'd also have a right under Pennsylvania law (23 Pa.C.S.A. Section 6102) to charge him with violating the Protection from Abuse Act which specifically prohibits somebody from following another person without proper authority.
I hope this is enough to get you started. Please feel free to call me if you need further assistance. I am a practicing Pennsylvania lawyer in Lehigh County and surrounding areas. -- Mark S. Guralnick - 1-800-LAWYERS or 1-866-337-2900.