(a) Whoever is present on the premises of any public or private school at any time, whether on the school grounds or in a school building, without lawful purpose or authority, shall be fined not less than $300 nor more than $1,000 and be imprisoned for a mandatory minimum term of six months. A sentence for conviction on any other count for the same incident of unauthorized presence, such as vandalism, theft or assault, shall be served consecutively rather than concurrently. Notwithstanding any other provision of law with respect to any person sixteen years of age or over who is found to have violated this section, adjudication of guilt or imposition of sentence shall not be suspended, deferred, or withheld, nor shall such person be eligible for parole prior to serving the mandatory minimum term of imprisonment prescribed by this section.
(b) In cases where a person under 16 years of age is found to have violated this section, the sentencing court may, in addition to such other disposition as it may order, require the violator to perform manual labor on the school premises involved, which labor may include grasscutting, painting, litter gathering, janitorial and other duties, for such period of time as the court deems appropriate.