Classmate no Okaa-san -Immoral Teacher-

Jun is a lovesick student at Seikyo School. One of his classmate's, Sae, has the hots for him. But Jun has a taste for the forbidden, ripened fruit of Sae's mother Keiko, who also happens to be their teacher. Little does he know that he's about to sample the best of both worlds. Whether he's satisfying Sae's sexual curiosity or satisfying his own with the experienced Keiko, Jun always gets an A for effort!

[From AniDB]