Well, as you might guess, the anime "Miyuki"（みゆき）and the pop band H2O are tied at the hip. Along with almost all of the other entries on the duo with the molecularly-inspired name, here is another single (their 6th from October 1983), "Goodbye Season" which was used as the ending theme for the show.
Compared to the other ending theme, the wistful and graduation ceremony-friendly "Omoide ga Ippai"（思い出がいっぱい）, "Goodbye Season" is more upbeat if still somewhat bittersweet. It's centered on the end of the summer season when the holidays are about to come to an end along with any romantic entanglements. Written by Keisuke Yamakawa（山川啓介）and composed by Kisaburo Suzuki（鈴木キサブロ）and arranged by Katsu Hoshi（星勝）, there is something about the delivery of Kanji Nakazawa and Masaki Akashio（中沢堅司・赤塩正樹）that brings out the goosebumps of nostalgia in me.
Not sure which original album, the song appeared on, but it is also available on any of H2O's BEST compilations. By the way, I gotta say that the ending credits here are gorgeous.