Failing grades, poor scores, inadequate results, disappointing outcomes or unacceptable performances are different synonyms that can be used in place of "bad marks". These terms indicate a lack of success or an unsatisfactory level of achievement in academic or non-academic pursuits. Other synonyms that can be used depending on the context include substandard, unsatisfactory, deficient or below-average. Using these different terms can convey a different tone or emphasis on the severity of the situation. Regardless, it is important to address the issue and work towards improvement in order to achieve better results.