Globally true magnitude of child sexual abuse is hidden because of its sensitive and illegal nature. Most children and families do not report cases of abuse and exploitation because of stigma, fear and lack of trust in the authorities. This study aims at establishing the effect of child sexual abuse on school performance among secondary school students in Mavoko Sub County, Machakos County. The objectives of this study were to examine the forms of CSA, the effect of CSA on school performance, social behaviour of CSA and school retention rate of CSA. The study was based on the descriptive-survey design. The target population was 3486 form three students divided into 14 strata that is corresponding to each of the 14 public secondary schools. Only 10% of students in form three (3) per school were targeted and to make a sample size be 84. The data was collected using semi structured questionnaires and interviews to school’s principals which were analysed using the Statistical Package for the Social Sciences (SPSS) version 24. Descriptive statistics such as: frequencies, percentages and means were calculated. Inferential statistics such as (Multiple Regression Analysis) were used to test the relationships between the independent (academic performance, social behaviour performance and school retention) and the dependent variables (forms of sexual abuse). The findings of the study show that there was a statistically significant relationship between the level of sexual abuse and the practice of various forms of sexual abuse (r=0.872, p<0.05). The most common forms of sexual abuse were verbal sexual abuse, involuntary kissing, or being sexually touched. There were also cases of attempted oral, anal or vaginal penetration. Further, some students were forced to watch pornographic pictures, drawings, films, videotapes, or magazines The findings also show that there was a significant relationship between sexual abuse and school social behaviour performance (r=0.909, p<0.05). Sexual abuse led to various negative school social behaviours. It meant that some of the students did not trust other people, felt lonely, and experienced stressful inner tension. It also led to mood rapidly changing in terms of anxiety, anger, and depression. There was a significant relationship between sexual abuse and academic performance (r=0.920, p<0.05). Due to sexual abuse, some of the students did not feel interested in classwork and, did not concentrate on class activities. Most of them were also not happy with academic performance and were always absent-minded in class. Pearson correlation analysis also showed a significant relationship between sexual abuse and school retention (r=0.933, p<0.05). Sexual abuse made some students feel that nobody cared about them in school and some decided to drop out. Some felt vulnerable and felt out of place in school. They never felt at ease in school and often decided to drop out. There were instances of students dropping out of school after being sexually abused due to pregnancy, stigma, early marriages, and stigma among others. The study recommends the need to put in place measures aimed at educating students on some of the disregarded forms of sexual abuse. The school administration should promptly identify and curb the tolerance of some of these forms of abuse among students. Counselling measures should be enhanced to check the negative psychological effects of child sexual abuse such as negative school social behaviour. Students who were victims of sexual abuse should be encouraged to remain in school.