छात्र्यादयः शालायाम्

Operational rule vidhi

The words छात्रि etc. get acute accent (उदात्त) on the first syllable when followed by the word शाला.

Word by word (padaccheda)

Substitute / term छात्र्यादयः prathamā · bahuvacanaRight context शालायाम् saptamī · ekavacana

Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
Right context Saptamī (7th case): "when X follows", the operation applies to what precedes (1.1.66 तस्मिन्निति निर्दिष्टे पूर्वस्य). Also a condition or domain ("in the sense of").

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Full reading

छात्र्यादयः शालायाम्

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

Commentary

The words छात्रि etc. get acute accent (उदात्त) on the first syllable when followed by the word शाला।
Thus छा꣡त्रिशाला, ऐ꣡लिशाला, भा꣡ण्डिशाला ॥

Where the Tatpurusha compound ending in शाला becomes Neuter, by the option allowed in 2.4.25; there also in the case of these words, the acute falls on the first syllable of the first term; thus superseding 6.2.123 which specifically applies to Neuter Tatpurushas. Thus छा꣡त्रिशालम्, ऐ꣡लिशालम् ॥

1 छात्रि, 2 पेलि (ऐलि), 3 भाण्डि, 4 व्याडि, 5 आखण्डि, 6 आटि, 7 गामि (गौमि) ॥

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.