There is a typical construction that I do not know specially when concerning about movements, in which we use the preposition y instead of the subject?
Russian prepositions can express either motion/tramsition, or state (both evidential and non-evidential).
The stumbling block is usually how to distinguish between a motion (or transition) and a state.
As prepositions, both на and в can be used to express either 'a translative/motional or 'an existential' state.
The difference is expressed by a case ending (and a case).
Locative (AKA 'Prepositional') is normally used with 'existential' state, while Accusative suggests motion or transition. Cf:
Я на химии (existential, Feminine Locative, non-evidential).
Я на химию (motion, Feminine Accusative, non-evidential).
Suppose there is a room called 'химия' between pals of a chemistry lab. A guy willing to express his literal acts or states would use phrases like:
Я в "химию" (motion, Feminine Accusative, evidential)
Я в "химии" (existential, Feminine Locative, evidential)
In Russian, a possessive structure (e.g. phrases like 'I have', 'I own', etc.) is always a structure of state. Hence, it is possible to say
После русского языка я (буду) на лекции / на химии
После русского языка у меня будет химия
После русского языка я - на химию
but not * После русского языка я буду в лекцию химия.
The phrase у меня есть means possession = я имею (I have). Your sentence contains this phrase in the future tense у меня будет (I will have, I am having). The sentence should be:
После русского языка у меня будет химия (лекция по химии).
You can use another verb to describe your action.
После (урока) русского языка я пойду на лекцию по химии.
("Я буду на лекции" is understandable too, but sounds a bit clumsy.)
The following article explains your subject rather well:
Your example, in particular, has a direct English analogy: I have Chemistry after Russian sounds quite regular.