Effects of student grouping on academic achievement were examined in research among high school students in Israeli kibbutz schools, and in a nationwide sample of middle school students in Israel. Data analysis was based on a model developed from research findings in the United States, Sweden, Great Britain, and Israel. The treatment variable was conceptualized as the quality of the socio-learning environment (SLE) resulting from segregating students with different levels of personal learning resources, scholastic ability, and motivation, in homogeneous classes of "high" and "low" ability, or in heterogeneous classes. In the kibbutz sample, no learning advantage was found in homogeneous classes. A slight advantage was observed in heterogeneity, concentrated mainly in the lower half of the class distribution. In the middle school sample, the class intellective composition (measured by class mean achievement on objective tests) emerged as the central factor in SLE quality, while ethnic and socioeconomic factors were weak determinants. A positive effect of SLE on objective academic achievements was indicated. In both samples, the loss of the "lows" under segregation was greater than the gain of the "highs," while under heterogeneity, the loss of the "highs" was smaller than the gain of the "lows." Effects of motivation factors on SLE effects were also investigated. (Author/MJL)