Failing Algebra

After Mrs. Jacobs' five children received their test scores, she asked them what their test scores were.

Her first child said that at least one of them failed algebra. 
Her second child said that at least two of them failed algebra. 
Her third child said that at least three of them failed algebra. 
Her fourth child said that at least four of them failed algebra.

If only one child is telling the truth, can you determine how many of Mrs. Jacobs' children failed algebra?