When you specify a new constructor then you lose the default constructor that you originally had. The only way to get a default constructor back is to make a new one that takes no parameters. Eg
public Classe(){
enfants = new Eleve[30];
enfants[0] = new Eleve("enfant1","1");
for(int i = 0; i < enfants.length; i++)
{
enfants[i] = new Eleve("enfant" + i.ToString(), i.ToString());
}
}
hope this helps.