This is the 12023rd most frequent Polish word.
lekcyjnymi
"Lekcyjnymi" means "lesson-related" or "pertaining to lessons" in English.
In this case, 'lekcyjnymi' modifies 'ćwiczeniami', illustrating that the exercises were part of a lesson.
Podręczniki z lekcyjnymi ćwiczeniami leżały na stole.
The textbooks with lesson exercises were lying on the table.
Here, 'lekcyjnymi' refers to 'materiałami', specifying that the materials are connected to lessons.
Mieliśmy do czynienia z lekcyjnymi materiałami edukacyjnymi.
We dealt with educational materials pertaining to lessons.
Here, 'lekcyjnymi' is used as an adjective agreeing in number and case with 'tematami', describing the themes as related to lessons.