Mi profesora dice que una alumna concentrada comete menos fallos en el dictado.

No. Mi does not change for masculine vs feminine.

• mi profesor = my male teacher
• mi profesora = my female teacher

It only changes for number:

• mi profesora = my teacher
• mis profesoras = my teachers

So mi agrees with the thing owned in number, not in gender.

Because profesora refers to a female teacher.

In Spanish, many nouns for people change form depending on gender:

• profesor = male teacher
• profesora = female teacher

So mi profesora means my female teacher.

Spanish often leaves out subject pronouns because the verb ending already tells you who the subject is.

• dice = he/she says
• digo = I say
• dicen = they say

So Mi profesora dice... already clearly means My teacher says...
Adding ella is possible, but usually unnecessary unless you want emphasis or contrast.

Here que means that and introduces a subordinate clause:

• Mi profesora dice que... = My teacher says that...

In English, that is often optional:

• My teacher says a focused student makes fewer mistakes
• My teacher says that a focused student makes fewer mistakes

In Spanish, que is normally needed here.

Alumna means female student / schoolgirl / female pupil.

• alumno = male student
• alumna = female student

So una alumna concentrada means a focused female student.

If the sentence were talking about a male student, it would be:

• un alumno concentrado

Because alumna is a feminine noun, so it takes the feminine article una.

A learner may wonder about nouns beginning with a, because some feminine nouns use el or un in the singular, like:

• el agua
• un águila

But that only happens when the noun begins with a stressed a sound.

Alumna does not have stress on the first a. The stress is on lum:
a-LUM-na

So it keeps the normal feminine article:

• una alumna
• la alumna

Because the adjective agrees with alumna, which is feminine singular.

• una alumna concentrada = a focused female student
• un alumno concentrado = a focused male student
• unas alumnas concentradas = focused female students

In Spanish, adjectives often change to match the noun in gender and number.

Because in Spanish, descriptive adjectives often come after the noun.

So:

• una alumna concentrada = a focused student

That is the most normal order here.

If you put the adjective before the noun, it can sound more literary, emphatic, or less neutral. For a straightforward description, noun + adjective is the standard pattern.

Literally, yes, but in natural English here it usually means focused or paying attention.

So:

• una alumna concentrada = a focused student

Spanish uses concentrado / concentrada very naturally for a person who is mentally focused.

Yes. Spanish commonly uses cometer with mistakes, errors, crimes, etc.

• cometer un error
• cometer fallos
• cometer un delito

So comete menos fallos literally means commits fewer mistakes, which is perfectly natural Spanish.

In English, make fewer mistakes sounds more natural, but Spanish prefers cometer in this kind of sentence.

Both can mean mistakes or errors, but fallo is very common and natural in Spain.

Roughly speaking:

• error can sound a bit more formal, objective, or technical
• fallo is very common in everyday Spanish in Spain

So comete menos fallos sounds very natural in Peninsular Spanish.

You could also say:

• comete menos errores

That would also be correct.

Because menos is the normal word for fewer / less when talking about quantity.

• menos fallos = fewer mistakes

Menor / menores usually means smaller / lesser in a different sense, not simply a smaller number.

So:

• comete menos fallos = correct
• comete menores fallos = not the natural meaning here

For countable things like mistakes, English prefers fewer, but Spanish still uses menos.

It means in the dictation or in the dictation exercise.

In a school context, dictado is a common exercise where the teacher reads something aloud and students write it down.

So the sentence means the student makes fewer mistakes during that dictation exercise.

Because Spanish often uses the definite article where English might or might not use one.

Here el dictado probably refers to a specific dictation activity, such as the one they are doing in class.

So:

• en el dictado = in the dictation / in the dictation exercise

Using el sounds natural because it points to the known classroom activity.

Because dice que normally introduces something presented as a statement or fact, so Spanish uses the indicative:

• Mi profesora dice que una alumna concentrada comete menos fallos...

That is a normal factual report.

You would expect the subjunctive in other kinds of structures, especially with doubt, denial, or influence. For example:

• No creo que cometa menos fallos = I don’t think she makes fewer mistakes

But with dice que in a straightforward statement, the indicative is the usual choice.