Part 2 – Answering your question
"Kaya naman pala nagkalason-lason ang mga mag-aaral noong nakaraang taon" means, “So, that’s why the students GOT POISONED last year”.
“Lason” is the noun we use for “poison”. The most common ways to form the verb would be to use the affixes: “ma” (MAlason = to get poisoned); “in” (lasunIN = to poison – object-focused); “um” (lUMason = to poison – subject-focused); and “mag” (MAGlason – to poison oneself).
“Nagkalason-lason” comes from the “malason” form of the verb. In your translation, to say “...were poisoned...”, i.e., they were intentionally poisoned, would be to use the verb “lasunin”.
The conjugation of “malason” would be “nalason” (past), “nalalason” (present), and “malalason” (future). “Nagkalason-lason” is formed from the past tense “nalason”. The present tense would be “nagkakalason-lason” and the future tense, “magkakalason-lason”.
When do we use these “magka + root word-root word” forms? More by choice and not as a rule, we use them when the subject is plural and we would like to emphasize that the verb happened independently to each subject. The required repetition of the root word adds a sense of plurality to the meaning.
“Ang mag-aaral ay NALASON” = The student got poisoned. (singular)
“Ang mga mag-aaral ay NALASON” = The students got poisoned. (plural) – This just states that the students got poisoned altogether.
“Ang mga mag-aaral ay NAGKALASON-LASON” = The students got poisoned. (plural) - This also states that the students got poisoned, but in addition to that, it may mean that the degree of poisoning was different for each one, and/or that they got poisoned at different times, etc. That is, the way each student got poisoned was independent from and not necessarily the same as the rest.