The following are great shade trees for Southern California school campuses, suggested by Frank McDonough, Horticulturist and Botanical Information Consultant at The Los Angeles County Arboretum and Botanic Garden.

Deciduous Shade Trees

Mexican SycamoreMexican Sycamore / Platanus mexicana
This newly introduced tree is Mr. McDonough's first choice for shade at Southern California schools.  "The best bet for lots of shade in the minimal amount of time."  Can grow to 50-80 feet tall and over 30 feet wide.  A good choice where you want the maximum amount of shade for your investment.  Does well in almost all Southern California climates.  If irrigated properly, root damage is minimal for a tree of its size.  The Mexican Sycamore is less susceptible to the anthracnose leaf fungus and is considered "water saving."
[The former top choice in this category, the London Plane Tree (a relative of this species), is also very good for shade but has fallen out of first place due to its pollen being moderately allergenic.] 
Additional information - Frank McDonough, Botanical Info. Consultant, L.A. Arboretum (626) 821-3236  

Maidenhair TreeMaidenhair tree / Ginkgo biloba
Although slow growing at first, Ginkgo's hardiness in urban situations makes it well worth the wait.  Besides growing to 80 feet in height and almost 60 feet in width, this tree puts on a spectacular fall foliage show.  Plant only the male clones.  (Female trees produce a foul-smelling fruit).  Additional information - UConn Plant Database
Silk TreeSilk tree / Albizia julibrissin
Provides a broad, umbrella-like canopy of finely cut leaves.  Its shade is somewhat less dense than other shade trees, so it lends itself to shading areas where more light might be required.
Produces beautiful pink pom-pom flowers in the summer.  Be sure to get a variety resistant to "wilt" caused by the fungus, Verticillium.

Monterrey OakMonterrey Oak / Quercus polymorpha
Also known as Mexican Oak or Mexican White Oak, this drought tolerant deciduous oak, native to Mexico and south Texas, grows 2 1/2 ft/yr to a mature height of 45 feet.  This oak is not known to be susceptible to a new pest, the polyphagous shot hole borer, which is killing east coast oaks (English, Red, and Pin Oaks) in southern CA. 
Additional information - UConn Plant Database
Sawleaf ZelkovaSawleaf Zelkova / Zelkova serrata
Vase shaped moderate sized shade tree.
Zelkova is a Japanese relative to the American elm, but is less susceptible to diseases that make American elms a poor choice.
Green Vase variety is fast-growing to 70 feet.
Additional information - UConn Plant Database

Mr. McDonough adds...
The above trees are all deciduous species.  There will be cleanup maintenance in the fall when all the leaves are dropped, but for most school-based applications this would result in less overall maintenance compared with the relatively constant messiness of evergreen trees.  Considering the size of the shade canopy, there will be less problem with surface root damage with the recommended deciduous trees than with most evergreens.  Of course, there will be no significant shade provided when a deciduous tree is bare, but when it leafs out again in the spring, these trees do tend to produce a better shade canopy.

Evergreen Shade Trees
In some situations, year-round greenery might be preferred.  For example, a few evergreen trees might be sprinkled in among a predominantly deciduous planting for a more aesthetically pleasing landscape design.  Density of shade, growth rate, messiness, and surface root damage were all taken into consideration in the following three recommendations from Mr. McDonough.

Fern PineFern Pine / Podocarpus gracilior
Moderate growth rate to 30 feet. 
Quite messy but excellent near foundations with little risk of surface root damage. 
Mr. McDonough's first choice evergreen shade tree.

Carrot WoodCarrot Wood / Cupaniopsis anacardioides
A very beautiful evergreen tree.
Moderate to slow grower to 35 feet. 
Subject to root rot with heavy water so not good in lawns. 
Produces fruit which can be messy. 
Constantly sheds leaves.
Australian WillowAustralian Willow / Geijera parviflora
Not as dense a shade canopy as others above. 
Slow to moderate growth. 
Resents heavy watering (don't plant in lawn area.) 
Somewhat messy.

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