Learn Hardcore German: Die Mitschülerin sagt ehrlich, dass sie die Noten ungerecht findet. - The female classmate says honestly that she finds the grades unfair.

Schülerin = female pupil / student (at school), in general.
Mitschülerin = a female fellow pupil, i.e. another girl/woman who goes to the same school or is in the same class/group.
Klassenkameradin also means female classmate, but it emphasizes being in the same class (same Klasse) more strongly.
So Mitschülerin focuses on “co‑student / fellow pupil,” while Schülerin just says “female student,” and Klassenkameradin is “female classmate.”

Why is Mitschülerin capitalized, but ehrlich and ungerecht are not?

In German, all nouns are capitalized. Mitschülerin is a noun, so it must start with a capital letter.
ehrlich and ungerecht are adjectives/adverbs, so they are not capitalized in normal running text.
Adjectives only get a capital letter if they are turned into nouns (e.g. das Ehrliche, das Ungerechte).

What does the -in ending in Mitschülerin tell me?

The ending -in is the regular feminine marker for many professions and roles:

der Mitschüler → die Mitschülerin
der Lehrer → die Lehrerin
So Mitschülerin specifically means a female fellow student.
The plural feminine form would be Mitschülerinnen.

In sagt ehrlich, is ehrlich an adjective or an adverb, and what exactly does it modify?

Here ehrlich works as an adverb and modifies sagt. It describes how she says it: she says it honestly / frankly.
In German, adjectives and adverbs have the same form, so we use context to see the function.
Because ehrlich is not directly in front of a noun here, it is not attributive; it is describing the manner of speaking.

Could ehrlich be placed somewhere else in the sentence, and would that change the meaning?

Natural options include:

Die Mitschülerin sagt ehrlich, dass … (most neutral)
Die Mitschülerin sagt, dass sie die Noten ehrlich ungerecht findet. (now ehrlich emphasizes her evaluation)
Die Mitschülerin sagt, ehrlich gesagt, dass sie … (using the fixed phrase ehrlich gesagt = honestly speaking)

Moving ehrlich can slightly change what feels emphasized (the honesty of her speaking vs the honesty of her opinion), but all versions keep the general idea of her being honest.

Why is the verb findet at the very end of dass sie die Noten ungerecht findet?

Because dass introduces a subordinate clause. In German subordinate clauses, the conjugated verb goes to the final position.
So the normal main‑clause order Sie findet die Noten ungerecht. becomes …, dass sie die Noten ungerecht findet.
This verb‑last pattern is standard after dass, weil, wenn, ob, etc.

What would the sentence look like without dass and with a separate main clause?

You could say, for example:

Die Mitschülerin sagt ehrlich: Sie findet die Noten ungerecht.

Here Sie findet die Noten ungerecht. is now a main clause, so the verb findet comes in second position, not at the end.

What case and number is die Noten, and how do I know?

Noten here is plural: singular is die Note, plural is die Noten.
The phrase die Noten is accusative plural, functioning as the direct object of findet.
You can tell it is accusative because in the pattern jemand findet etwas (Adj.), that etwas is what is being judged/found, i.e. the direct object.

Why doesn’t ungerecht have an ending like ungerechte or ungerechten?

ungerecht here is a predicative adjective: it describes the state of die Noten after a verb like finden, sein, werden.
Predicative adjectives in German do not take endings:

Die Noten sind ungerecht.
Sie findet die Noten ungerecht.
Adjective endings (like ungerechte Noten) appear when the adjective is directly in front of the noun as an attribute.

In dass sie die Noten ungerecht findet, who does sie refer to?

In normal context, sie refers back to die Mitschülerin.
German often uses a pronoun in the subordinate clause to avoid repeating the full noun:

Die Mitschülerin sagt, dass sie … = The (female) classmate says that she …
If the context were ambiguous (another feminine noun nearby), Germans might repeat the noun or rephrase to make it clearer.

Can I say Die Mitschülerin sagt ehrlich, die Noten sind ungerecht. without dass?

You can say that in spoken German, but it sounds colloquial and slightly “looser.” Strictly standard German would expect either:

Die Mitschülerin sagt ehrlich, dass die Noten ungerecht sind. (with dass)
or
Die Mitschülerin sagt ehrlich: Die Noten sind ungerecht. (with a colon or clear pause).
So for careful writing or exams, prefer the version with dass or the version with a clear second main clause.

Why do we use findet here instead of just saying Die Noten sind ungerecht?

Using findet makes it clear that this is her opinion, not an objective fact: she finds the grades unfair / she thinks the grades are unfair.
Compare:

Die Noten sind ungerecht. → presents it as a statement about reality.
Sie findet die Noten ungerecht. → clearly marks it as her personal judgment.