The word incursion according to The Australian Oxford Dictionary , Second Edition, means 'an invasion or attack, especially when sudden or brief'.
Now when a bunch of school children all board a tram at once it certainly feels like an incursion, but in this case schools are using the word to mean an excursion which takes place at the school. That is, 'in' is replacing 'ex' to mean of an activity held at the school, as compared with outside of the school.
The online Oxford dictionary makes no reference to incursion with regards to an educational activity. The Australian Oxford Dictionary doesn't make such a reference either. However, the Macquarie dictionary does include the additional meaning for incursion as, 'an educational activity in which an artist, educator, etc., visits a school to give a demonstration, etc., of their specialty'.
Could it be that what was once a simple play on words has become another meaning for the word?
Care should be used when using the word referencing an internal activity, because that usage would normally not be considered correct.
What is interesting, is we now have a generation of students who have grown up using the word and when that happens, and enough people know the alternate meaning for a word, that word and the new meaning becomes part of the vernacular.
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