the problem is that the modifying phrases are dangling. they are not next to the noun they are modifying and hence the meaning is unclear.

in the first sentence, it sounds like the class has long brown hair and brown eyes, not the girl.

in the second sentence, the venom is secreted and stored in the teeth, but it sounds like it’s saying that happens in the glands.

see what i mean, jelly bean?

1. In this sentence who has long brown hair/eyes the class or the girl? It should read “In our class the girl with the long brown hair and eyes is the prettiest or The girl with the long brown hair and eyes is the prettiest one in our class.

2. Same problem plus an incomplete sentence – where is the venom..in the teeth or the glands? Venomous snakes, with modified teeth, which are connected to poison glands where the venom is secreted and stored….

2. not sure, since the venom is secreted by and stored in the glands, not the teeth. I don’t see a problem with this phrase, unless you need a comma before “in which.” This would be a punctuation error, not a grammatical error.

2, Venomous snakes with modified(modified, wrong word)

fangs connected to poison glands, in which the venom is stored and secreted.

teeth is also wrong word, fangs, is right.