Aim: Teachers are uniquely positioned to shape students’ motivational beliefs, yet research examining perceived classroom messages that explicitly convey the value of what students are learning remains scarce. Background: Drawing on the situated expectancy value theory, we examined how student-perceived teacher messages about the value of science longitudinally related to adolescent students’ subjective task values, perceived costs, and expectations of success, achievement, and aspirations. Sample: Data were collected from 1591 participants aged 11 to 14 years (57.5% female) nested in 65 science classes in England. Method: Data were collected over four waves. Perceived teacher value messages were collected in the first wave, achievement and aspirations in the second and fourth waves, and expectancy, utility value, and effort cost in the first and third waves. Results: Class-aggregated perceived teacher value messages positively predicted achievement mediated by utility value, and positively predicted aspirations mediated by expectancy and utility value; class-aggregated effort cost negatively predicted aspirations. Additionally, student-level expectancy and utility value showed positive reciprocal relations with achievement and aspirations; student-level effort cost negatively predicted achievement and aspirations but not vice versa. Conclusion: When students perceive that teachers explain the value of science, they report greater expectancy and utility value, and show subsequent higher achievement and aspirations.