Canon Law states: “Holy communion is to be given under the species of bread alone or, in accordance with the liturgical laws, under both species or, in case of necessity, even under the species of wine alone.” (Can. 925)
Theologically, both the Sacred Host and the Precious Blood communicate the “whole Jesus” to the recipient. For many years in England and Wales it has been permitted, (and indeed encouraged), for not only the celebrating priest(s) but also others who assist at the Mass, to receive Holy Communion under the forms of both bread and wine.
However the “liturgical laws” that the Canon refers to specify that communion under both kinds should only happen when a) there is no danger of irreverent treatment of the Precious Blood and b) where communion in both kinds would not unduly lengthen the celebration of the Mass.
Because in many larger school Masses these safeguards need to be ensured, our custom is to have communion under the species of bread alone. However, on certain occasions (such as Corpus Christi and the Sacred Heart), it is highly appropriate for all to be offered the opportunity to receive under both species.
The celebrating priest (and any concelebrants) must receive in both kinds to complete the Eucharistic Sacrifice.