This study examined the relationships between students' perceptions of teacher support, the social classroom environment, school loneliness, and possible gender differences among 2099 first year upper secondary school students in Norway. Data were collected in the fall (t1) and spring (t2) of the school year. Results from structural equation modelling (SEM) analyses showed that perceived emotional and instrumental teacher support were directly related to students' perceptions of the social classroom environment, and indirectly to student loneliness through the social classroom environment. While for boys, both types of teacher support were significantly related to these variables, only emotional teacher support was of significance to girls. The strongest contributing factor to students' school loneliness was their perceptions of the social classroom environment. Some implications of this study are that a positive social classroom environment is an important safeguard against student loneliness, and that teachers can aid in preventing loneliness among students through facilitating a positive social environment in the class.