The purpose of the current study was to examine differences in multidimensional perfectionism, help-seeking, negative affectivity, and social-emotional well-being between grade 9 to 12 early college high school students who received the modified version of Cognitive Behavioral Therapy for Perfectionism (CBT-P) small group counseling intervention (Egan et al., 2014a) and grade 9 to 12 early college high school students in the comparison group who did not receive the intervention. Masters-level counselors in training (CIT) implemented CBT-P with early college high school students after being trained in the use of the program and other study-related procedures. The study followed a quasi-experimental, non-equivalent pre-post design and employed various self-report measures (DASS-21, SEHS-S, CAPS, and GHSQ). A series of ANCOVA analyses were conducted to determine statistically significant differences between the treatment and comparison groups. The researcher reported partial eta squared [partial eta-squared] effect size for each independent variable. Results of the study revealed a statistically significant difference in negative affectivity and self-oriented perfectionism between the treatment and the comparison group. However, no statistically significant difference, by treatment condition, was found regarding participants' socially prescribed perfectionism, help-seeking intentions, or social-emotional well-being. The modified CBT-P treatment has found large effects ([partial eta-squared] = 0.219) in reducing negative affectivity as measured by the DASS-21 and medium to large effects ([partial eta-squared] = 0.115) in reducing self-oriented perfectionism. This study provided clinical support for using the modified CBT-P small group intervention (Shafran et al., 2002) in early college high schools to decrease negative affectivity and perfectionism in students. Furthermore, the study further supports the importance of building social-emotional wellness to improve students' mental health. Finally, it highlights the need for future research to determine the impact of perfectionism and small group interventions on early college high school students' mental health, well-being, and help-seeking behaviors. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]