Mental distress is highly prevalent in university students, and autistic traits can hinder academic education. The substantial lifestyle changes experienced by new university students can induce mood and anxiety dysfunctions and subsequent suicide-related behaviors. The aims of this study were to evaluate the detectability of suicidal ideation using the Kessler Psychological Distress (K6) questionnaire, a short and easily administered measure, and to elucidate the relationship between K6 scores and developmental disorders following university admission in medical students. The mental states of 341 new students at University School of Medicine were prospectively evaluated using the short questionnaire including the K6 at admission and 6 months later. Participants were simultaneously assessed on the five core features of autism spectrum disorder (ASD), four features of attention-deficit hyperactivity disorder (ADHD), and presence of suicidal ideation on this short questionnaire. K6 scores were significantly higher 6 months after admission than at admission (p = 0.028). Students with suicidal ideation had significantly higher K6 scores than those without (p < 0.001), and K6 scores at admission differentiated students with suicidal ideation both at admission (area under the curve [AUC] = 0.947) and 6 months after admission (AUC = 0.913). K6 scores were positively correlated with scores of ASD- and ADHD-related items ([rho] = 0.451 and 0.440, respectively) and with autism-spectrum quotient scores (p = 0.041, [rho] = 0.501). In conclusion, the K6 questionnaire may be useful in efficiently detecting university students with suicidal ideation or autistic traits.