What is the ruling on congregational prayer when the Imam is in the mosque and most of the worshippers are outside in the schoolyard, or when the Imam prays in the street and the worshippers follow him from inside and outside the mosque?

The prayer of the congregant is valid if he moves ahead of the Imam due to an excuse, such as crowding or lack of space, because refraining from moving ahead of the Imam is one of the obligations of congregational prayer, and all obligations are waived due to an excuse. There is no harm in the Imam praying outside the mosque with the congregants behind him, especially if it is due to the mosque being too small.