Do Minsk schools "insure" before silent action?

The children have been obliged to bring the notes of the following contents, signed by their parents: "My son… on 21.09 since 16.00 till 21.00 will stay at home together with his parents". A father of the fourth form student of one of Minsk schools has informed this to “Nasha Niva”. They also threatened the kids that if someone failed to bring such note, he would not be let out of school until 21.00.