Thrice Holy. Orthodox services love to pepper this bad boy in whenever they can. You'll notice that the recording begins with a doxology, and you'll also hear that the priest is reciting prayers while the choir chants the hymn. Because it's such a short hymn, consider letting some students attempt those harmonizing notes, called ison while the rest of the class chants. Also consider switching between Greek and English.
Apologies for the quality of the recording. Because it's such a simple song, recording artists tend to be quite free with the rhythm and melody.
Ἅγιος ὁ Θεός,
Ἅγιος ἰσχυρός,
Ἅγιος ἀθάνατος,
ἐλέησον ἡμᾶς.
Holy God
Holy Mighty
Holy Immortal
Have mercy on us