School was back in session across most of New Jersey and so were the threats and heightened security measures that have become commonplace since the mass shooting at a Florida high school.

Between Wednesday and Friday, four students were charged with making threats in two school districts, while a 19-year-old man was arrested for sneaking into a Mercer County high school.

Hopewell Valley Central High School,  Timberlane Middle School, the Toll Gate Elementary School, the Pennington School and the Pennington Montessori School were all on lockdown Friday afternoon after a high school student let in Foster Krupa, 19, through a side door around 2:45 p.m.

Police said the student did not recognize Krupa but let him in to be "polite" because Krupa said he was looking to visit a friend.

A teacher who did not recognize Krupa confronted him and Krupa ran out the door, police said. School personnel chased Krupa on foot toward Timberlane as school was letting out for the day.

Police went to Krupa's Lawrenceville home and found him there about 3:15 p.m. He was charged with criminal trespassing and causing or risking widespread injury or damage and released by Hopewell police. The Mercer County Prosecutor's Office is investigating.

"Based on the police investigation, we have no reason to believe that this individual gained entry to CHS to cause harm. However, this incident underscores the importance of maintaining a secure campus. This includes not propping or opening an outside door for anyone," the school wrote on its website.

Secaucus High School and Middle School were also on lockdown Friday after a threat was made by two 14-year-old boys. Police Capt. Dennis Miller told NJ.com that no weapons were found after a search of the schools, which share a campus. The boys were both suspended and were charged with making a terrorist threat.

Two students were charged in separate Snapchat threats in Evesham on Wednesday and Friday, according to Evesham police.

A 15-year-old boy was arrested Wednesday for posting about his wish to "shoot up" Cherokee High School. Police said they took the boy into custody and later released him to his parents.  A 12-year-old middle school student was arrested on Friday after sending a message via Snapchat to another student about "wanting to plan a school shooting."

Both boys were charged with making terroristic threats and false public alarms.