Deputies say he left Oakland County Children's Village for a weekend visit home before he disappeared.
The sheriff's office says he does not have a history of violence or threats.
Greiwe is believed to be in the Orion Township area.
Lake Orion Community Schools also closed for programs Monday as a precautionary measure, with parents with children at the schools for programming asked to pick them up.
7 Action News has obtained the following letter sent to families by the district:
As you are aware, this morning I dismissed all children from summer programs, athletic practices, and other events held throughout the district as a result of a report from the Oakland County Sheriff related to a runaway teen who, reportedly, was in possession of a weapon and ammunition. And, as I'm sure you are also aware from the multitude of media reports, the teen is a former Lake Orion student; however, he has not been a student in our district for the past two years as he has been in Oakland County Children's Village, although scheduled to be released on Tuesday. Because he made mention that he was coming to the high school to "try out" for football, precautionary measures were taken today to ensure the safety of all children attending summer activities as well as staff members in the various school facilities.
As of this writing (6:30 p.m. Monday evening), the teen has not been located. The Sheriff Department has sent out this notice: "Caution should be used when this person is seen and anyone who sees him should call 911."
I am going to continue with closing summer programs, activities, and events until I am notified that this young man has been apprehended. While I know, and regret, that children will be disappointed they can't attend planned activities and I recognize the inconvenience to families, I am unwilling to take any chances that potentially could endanger children or adults in the district.
Because many parents will need to make child-care arrangements, I am notifying you this evening that all district activities, events, practices, etc. for Tuesday, June 20th are cancelled. In order to avoid confusion, in the event he is apprehended some time tonight, the cancellation will still remain in place on Tuesday.
I will continue to keep you updated.
Marion Ginopolis, Superintendent
No threats have been made at this time. The former student's whereabouts are unknown.