Welcome!
Without knowing what rating scale was used, it's a bit difficult to discuss its validity. Is it a norm-referenced standard published scale, or a school-created qualitative rating scale? Some of the scales are also wider ranging, with one segment on intellectual giftedness, and several other domains regarding other kinds of giftedness. Also the score is not meaningful without knowing the mean and standard deviation, and whether the norms are based on GT students or neurotypicals.
Schools tend to select or create scales based on the nature of the GT program. So, for example, if the program is designed around creative writers with strong written and oral communication skills, then the scales might help teachers identify those with performance strengths in these areas. On the flip side, teachers tend to rate students, as you note, somewhat subjectively, with a bias toward those who match their own teaching style.
When you say "on the cusp", do you know what norms were used with SAGES (or which edition; there are three)? There are two norms: the gifted norms and the general norms. An average score on the gifted norms typically is taken to strongly suggest that a child should be identified as gifted. A very superior score on the general norms is typically taken to mean the same thing.
