A few years back I received a question about the appropriateness of students in catechetical programs signing one another on the forehead with holy water at the start of class. The catechist had been told that blessings can only be given by someone who has authority over the person receiving the blessing (e.g., a parent to a child or a bishop to a member of his flock) and wanted to know if this was true.
We need to make a distinction between liturgical blessings and devotional blessings. The former are defined by the Church and set out in ritual texts (such as the Roman Missal or Book of Blessings); the latter are not defined, but should support and and extend the liturgical practices of the Church (cf. Catechism of the Catholic Church, no. 1675).
The Catechism says of blessings:
Sacramentals derive from the baptismal priesthood: every baptized person is called to be a ‘blessing,’ and to bless. Hence lay people may preside at certain blessings; the more a blessing concerns ecclesial and sacramental life, the more is its administration reserved to the ordained ministry (bishops, priests, or deacons). (Catechism of the Catholic Church, no. 1669)
There is nothing in that explanation that supports a hierarchical notion of blessings of persons over persons; the principle regards the nature of the blessing itself.
So the question then becomes: how concerned with “ecclesial and sacramental life” is a non-liturgical blessing on the forehead with holy water?
The Book of Blessings gives us some guidance. Generally, liturgical blessings over people (“Blessing of the Sick,” “Blessing of a Mother Before Childbirth, “Blessing of Students”) give the option of allowing a lay person to preside if no clergy are present. The text states that
[L]aymen and laywomen, in virtue of the universal priesthood, a dignity they posses because of their baptism and confirmation, may celebrate certain blessings… Such laypersons exercise this ministry in virtue of their office (for example, parents on behalf of their children) or by reason of some special liturgical ministry or in fulfillment of a particular charge in the Church, as is the case in many places with religious or catechists appointed by decision of the local Ordinary. (Book of Blessings, no. 18; emphasis in the original)
Students signing one another with holy water seems in line with the “Blessing of Students.” Given the principles of the Book of Blessings, the ideal would be for the catechist to do the blessings. (And in fact, I think it would be a powerful ritual symbol for a catechist to sign the foreheads of students as they came into the classroom.) But since the signing in question isn’t a liturgical action, but a devotional one, I don’t think there’s anything stopping a catechist from delegating the act of signing to a student. (If it were a liturgical action, such as celebrating the “Blessing of Students” from the Book of Blessings, I’d say the catechist must preside, unless a member of the clergy is present, in which case he would be the proper minister.)