Final answer:
The word 'spike' is used by the author to indicate a sudden and significant increase in student anxiety levels, observed over the past five years.
Step-by-step explanation:
The author uses the word "spike" to signify a sudden increase or surge, particularly referring to the rise in anxiety among students as observed by the teacher over a period of five years.
In the context given, "spike" indicates a noticeable and considerable upward trend in the levels of anxiety, implying that the issue has intensified rapidly and has become more pronounced or acute during that timeframe.