Especially in X "class" or "classes"?

There are several issues here.

(1) Which sense of 'class' is intended here?

• (a) a group of students who are taught together at school, college, or university:

...

She gave the whole class extra homework for a week.

[John is not in my maths class.]

[John and Jill are in different classes.]

or

• (b) a period of time in which students are taught something:

...

I missed my aerobics class yesterday.

[John missed his maths class yesterday.]

[John has missed all his maths classes this week.]

[Cambridge Dictionary, with added examples]

Obviously, both senses are used in count usages.

(2) Are reading classes and grammar classes distinct entities, or are 'reading and grammar classes' (like health and safety lectures) single entities?

(3) With count usages, the distributive singular is quite often a complication (so "Gentlemen, on your way home, buy your wife a bunch of roses" is fine).

............................

(1) We'll assume CE sense 1b, the teaching period, as being more likely, though the ambiguity needs resolving.

(2) We'll assume that reading periods/classes and grammar periods are distinct.

(3) Here, though (as your grammar checker validates) the distributive singular is available, probably reflecting sense 1a but almost synonymous with 'course', I'd feel more comfortable with the formally obvious plural, classes. In the UK, 'especially in reading and grammar lesson' would be unacceptable, though 'especially in a reading or a grammar lesson' would be fine. As would 'especially in reading or grammar lessons'.