This is what Wikipedia has to say on the subject:
A locum, or locum tenens, is a person who temporarily fulfills the duties of another; the term is especially used for a physician or clergyman.For example, a locum tenens physician is a physician who works in the place of the regular physician when that physician is absent, or when a hospital or practice is short staffed. These professionals are still governed by their respective regulatory bodies, despite the transient or freelance nature of their positions.
The word locum is short for locum tenens, a Latin phrase meaning "place holder", akin to the French 'lieutenant'. The abbreviated form "locum" is common in Australia, Canada, Ireland, Malaysia, Singapore, New Zealand, South Africa, and the United Kingdom; unlike in Latin its plural is locums. In the United States, the full length "locum tenens" (plural: locum tenentes) is preferred, though for some particular roles, alternative expressions (e.g., "substitute teacher") may be more commonly used.
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I did wonder what the plural might be; I even thought it might be locos, the accusative plural of locus!
ps. In UK the usual term for a temporary replacement teacher is 'supply teacher'; this would refer to a teacher outwith the school's normal staff brought in from outside.
For a very short absence, the teacher's classes would normally be covered internally by his/her colleagues.