Clearly, diction, the choice of words, sets the tone of sentences. So, in selecting words to create a more positive tone, the writer must consider both the denotation (literal definition) and the connotation (what meanings are suggested) of words.

With respect to the word smart, for instance, there are differing denotations such as mentally quick, intelligent, clever, shrewd.  Connotations of this word include being capable of turning situations to one's advantage and resourceful and clever more than having high innate mental capabilities. Therefore, in order to give the sentence a more positive tone, perhaps the writer can use what the editor has noted, and write

The shrewd and mentally quick student earned As and Bs.

First, before changing tone, identify clearly what the bold words are saying. The tone here is semi-positive; it's a little more positive than neutral. Clearly what the bold words indicate is that a well respected student scholar received admirable marks on their (non-gender singular "their")assessments of academic achievement.

In order to make a boldly positive tone, the degree of respect and admiration must be increased. Similarly, a negative tone requires a significant reduction of respect and admiration. Start with synonyms.

Smart: some of the many synonyms are acute, astute, egghead, crafty, clever, bright, fresh, shrewd, keen, and ingenious. Some of these that lead to positive tone might be clever and bright. Some that lead to negative tone might be egghead and shrewd.

Grades: some synonyms are level, mark, rung, standard, degree. Choices here are fairly neutral; almost any synonym listed here can be used with either positive or negative tone (see more synonyms at thesaurus.com). A choice for a more positive tone, though, might be mark or level. A choice for a more negative tone might be rung or standard.

Received: this verb means, among other meanings, to have bestowed or conferred. These are synonymous meanings. Other synonymous words or phrases are to be burdened with or to sustain, accept, acquire, collect, derive, draw . A choice for a more positive tone might be conferred or bestowed. A more negative tone might come from draw or burdened.

Student: some synonyms are amateur, beginner, neophyte, probationer, scholar, abecedarian. A positive tone might come from scholar, while a negative tone from probationer, abecedarian, neophyte.

Negative tone example: The shrewd probationer drew good marks.

(More on most of these at thesaurus.com)

The more positive tone could focus on positive attributes of the student like: hardworking, gifted, honest, disciplined, determined, successful.  For verbs, consider: earned, achieved, scored.  The hardworking student earned good grades. 

The more negative tone should likewise focus on more negative attributes of the student:  lazy, undisciplined, dishonest, disrespectful.

The dishonest student stole good answers.

"Smart" can be changed to "nerdy" or "overachieving" to create a more negative tone. To generate a more positive tone, "smart" can be changed to "bright", "hard-working", or "intellectually dedicated". 

"Grades" is a more difficult term to substitute, but you may think about terms like "rewards", "results", "marks", "report", "feedback" or "evaluations". 

You might make the sentence negative by saying: The smart-mouthed student had good grades.  This gives the word "smart" a negative connotation and changes the emphasis on the grades to "had" which seems more passive.

To make it positive, you could say: The conscientious student always earned good grades.  Adding the word "conscientious" instead of "smart" makes it more a positive behavior on the student's part.

To make it be more positive, you should make it more active on the part of the student -- more like he or she is actually doing something rather than sitting there and having the grades given to them.  You could say "the smart student earned good grades" or even "the brilliant student earned good grades."