If you have a major project you need to implement, I believe you are far better off applying for multiple grants to make sure you cover all your financial needs and give your new or improved program every chance of success. 

Writing multiple grants gives you several advantages:

1) The more quality applications you submit, the more likely you are to get at least one or two funded.

2) Completing a second, third, or fourth grant application gets easier and easier because you will essentially use the same data again and again, just in slightly different forms.

3) If you have a large project with large expenses, you may have to get grant money from several sources just to cover everything. 

4) Quality practice improves your application.  Your second, third, and fourth applications will probably be much better than your first.  You will be better able to describe your needs and be more convincing in your narrative.  You have to be very careful and stay constantly on guard to get your highest quality application the first time through.

If you already have time problems, you may not have the luxury of submitting several different grant applications.  However, there are many advantages to submitting multiple applications.

I do want to mention a couple of things you may be tempted to do but shouldn't.  It is a virtual waste of time to submit the same letter to foundation after foundation.  The person reading the letter will be able to tell what you're doing.  Applications for grant funding should be individualized and personalized based on the giving patterns of the foundation and the needs of your school.  Random letters seldom tell your story well or inspire foundation board members to support your programs.  Use the foundations own grant application form if they have one.  If not, follow their directions for applying by letter exactly and completely.

If you are seeking funds to buy particular commercial programs, never cut and paste their advertising material into your applications.  They may tell their story well, but your job is to demonstrate how the commercial product fits into the overall program you are developing.
Many grant readers view the use of advertising copy in a grant application as a lazy, impersonal way to get grant money.  You may want to use their copy; just put it in your own words and describe how the product will be used to benefit your students.

I firmly believe that multiple grant applications are the way to go when you are seeking grant funding.  The warning never to put all your eggs in one basket may be old and trite, but it applies today as well as it ever did.  Multiple grant applications may mean multiple streams of money for your school.

Don Peek is an expert in school funding. He has run The School Funding Center since 2001. Its database contains over 100,000 grants available to all types of schools in the United States. Don worked in education for 20 years as a teacher, principal, and assistant superintendent before becoming the VP then the president of the training division of Renaissance Learning, developer of the Accelerated Reader.
http://www.schoolfundingcenter.info