Here are some particularly promising sources of potential funding I’ve identified and their deadlines:
The Toyota TAPESTRY Program gives grants of $10K (and mini-grants of $ 2.5K) for hands-on science projects. January deadline.
Toshiba America Foundation “The Foundation strongly encourages projects planned and led by individual teachers or teams of teachers for their own classrooms. Many successful grantees have designed projects that tap into the natural curiosity of their students, enable students to frame their own scientific questions, and incorporate the expertise of community partners.” Deadline August and February for large grants (over $5K) – grants under $5K any time.BP's A+ for Energy Program gives grants of $5K - $10K for projects that teach about energy and energy conservation. Only certain counties in certain states are eligible. March deadline.
PG&E's Solar Schools Program for northern and cental California gives "Bright Ideas Grants" of up to $5K (March and September deadlines), 1kW photovoltaic systems (September deadline) and offers free one-day training for teachers on solar energy.
The Marin Municipal Water District gives Teacher Action Grants of up to $2000 to cover direct expenses for school garden projects in Marin County. There are similar programs for Contra Costa and Alemeda counties.
Lowe’s Outdoor Classroom Grant Program especially oriented to greenhouses, community gardens, etc. Only outdoor classrooms proposals. Early spring deadline, $2000 - $20K
The National Science Foundation chiefly funds university-level research, but sometimes there is an opportunity for high school involvement. See for example the Arctic Research and Education Program.
Motorola Innovation Generation Grants especially fund after-school and science summer enrichment programs. Spring deadline. Average grant size is $30K!
The Drake Fund (only for teachers at Sir Francis Drake High School, my institution) gives grants of a few thousand dollars. There is an early spring deadline.