New Cues and Solution
The word you're looking for is
protest
I start with the good
Pro, someone who is good at what they do (pro gamer)
and end with the (sort-of?) bad
Test, which can be considered bad, if it is very hard or you didn't study enough.
My second letters spell poet
protest
I rhyme with rest
Test rhymes with rest, so protest does as well.
Old Clues
I am a third like the fourth state of matter
That's PLASMA
I am a third like what math puns make me feel
Puns make me feel numb, but maths puns make me feel NUMBER
I am a third like the blank: C U IN THE _ _ _ _ _ _
There's a book called "See You in the COSMOS", the description of which sounds perfect for English class.
My idea:
All of these words have six letters, so a third of them would each be two letters.
If we take two letters from each word to form a new one, the new one will be 1/3 like each of the other words.
I tried splitting the words into two letter particles, those being:
"PL" "AS" "MA"
"NU" "MB" "ER"
"CO" "SM" "OS"
I tried combining these particles into new words and translating the result from Spanish to English (as per the next line).
The problem:
The only words google translator can translate are:
aseros - handles
manuos - manuals
mambos - mambos (dances)
erasos - erased
oseras - bear caves
comear - eat
None of these options make any particular sense. And my rudimentary knowledge of Spanish makes me doubt that some of them are even words.
I suspect I can't just cleanly slice the words into three to get the thirds but haven't had time to check every possible combination yet. Also, before I do that, I'd like some indication that I'm on the right track