Using first-principle calculations in conjunction with the frozen-magnon technique we have calculated the exchange interactions and spin-wave dispersions in the series of the zincblende half-metallic II-V (Caz, Z = N, P, As, Sb) ferromagnets. The calculated exchange constants are used to estimate the Curie temperature within the random phase approximation. The large Stoner gap in these alloys gives rise to well-defined undamped spin waves throughout the Brillouin zone. Moreover we show that the spin-wave stiffness constants for the considered systems are among the largest available for local moment ferromagnets. The predicted Curie temperature of half-metallic CaN is noticeably higher than the room temperature with respect to the other compounds, and thus we propose CaN as a promising candidate for future applications in spintronic devices.