If you have your instrument at home, just practice!(grades 4-12)
Check out Learn to Play Music.comon youtube, which has instructional videos for instruments ranging from recorder and ukulele to strings and woodwinds to piano.
Play along with Rockin' Recorder songs -- method book and tracks right on the website
Access vocal warm-ups, karaoke, and/or practice tracks on youtube(grades 3-12)
Use practice tracks in Google Classroom or on youtube for choral repertoire and/or solo works (grades 4-12)
Art Song Central has free sheet music for many classical songs in the public domain, and you can easily find practice tracks and accompaniments on youtube. The 24 Italian art songs are a great place to start.
Follow Sing Well, a 10-lesson online singing class available for free on "A Song a Week by Jodie".
Research the background of one of your pieces - composer, time period, culture, significance of the piece, translation of lyrics, etc. (grades 5-12)
Explore music history and genres through this interactive Music Timeline
Things to Watch
In the Berliner Philharmoniker's Digital Concert Hall, view over 600 concerts with their 30-day free subscription (which does not automatically renew)
The Met Opera is streaming encore presentations from their Live in HD series for free on their website (each production begins at 7:30 p.m. EDT and will be available until 3:30 p.m. the following day, beginning 3/16)
Check out the show "Broadway or Bust" free from KPBS, which follows 60 musical theatre students to New York to be coached for the National High School Musical Theatre Awards competition.
The competition show "Sing That Thing!" features a cappella choirs in numerous styles. Full episodes of all five seasons are available for free.