Income tax expense. Income tax expense was $10.3 million for the three months ended March 31, 2021, or 30.2% of income before income taxes, as compared to $4.4 million, or 33.5% of income before income taxes for the same period in the prior year. The decrease in the effective tax rate for the three months ended March 31, 2021 was primarily due to the impact of non-deductible compensation.
Net income. Net income was $23.8 million for the three months ended March 31, 2021, compared to $8.8 million for the same period in the prior year, representing an increase of $15.0 million.
Comparison of the Nine Months Ended March 31, 2021 and 2020
Revenues. Our revenues for the nine months ended March 31, 2021 were $1,139.3 million, representing an increase of $367.5 million, or 47.6%, from $771.8 million for the same period in the prior year. General Education revenues increased $250.8 million, or 35.9%, year over year. The increase in General Education revenues was primarily due to the 47.2% increase in enrollments, school mix (distribution of enrollments by school), and other factors. Career Learning revenues increased $116.6 million, or 160.8%, primarily due to a 127.3% increase in enrollments, school mix, as well as from the acquisition of Galvanize.
Instructional costs and services expenses. Instructional costs and services expenses for the nine months ended March 31, 2021 were $741.0 million, representing an increase of $225.2 million, or 43.7%, from $515.8 million for the same period in the prior year. This increase in expense was primarily due to the incremental personnel and related benefit costs associated with supporting higher enrollments. Instructional costs and services expenses were 65.0% of revenues during the nine months ended March 31, 2021, a decrease from 66.8% for the nine months ended March 31, 2020.
Selling, general, and administrative expenses. Selling, general, and administrative expenses for the nine months ended March 31, 2021 were $309.2 million, representing an increase of $78.6 million, or 34.1% from $230.6 million for the same period in the prior year. The increase was primarily due to an increase in personnel and related benefit costs and also an increase in professional services expenses. Selling, general, and administrative expenses were 27.1% of revenues during the nine months ended March 31, 2021, a decrease from 29.9% for the nine months ended March 31, 2020.
Income tax expense. Income tax expense was $18.5 million for the nine months ended March 31, 2021, or 23.3% of income before income taxes, as compared to $6.0 million, or 23.4% of income before income taxes for the same period in the prior year.
Net income. Net income was $61.0 million for the nine months ended March 31, 2021, compared to $19.6 million for the same period in the prior year, representing an increase of $41.4 million.
Liquidity and Capital Resources
As of March 31, 2021, we had net working capital, or current assets minus current liabilities, of $575.4 million. Our working capital includes cash and cash equivalents of $329.0 million and accounts receivable of $422.8 million. Our working capital provides a significant source of liquidity for our normal operating needs. Our accounts receivable balance fluctuates throughout the fiscal year based on the timing of customer billings and collections and tends to be highest in our first fiscal quarter as we begin billing for students. In addition, our cash and accounts receivable were significantly in excess of our accounts payable and short-term accrued liabilities at March 31, 2021.
During the first quarter of fiscal year 2021, we issued $420.0 million aggregate principal amount of 1.125% Convertible Senior Notes due 2027 (“Notes”). The Notes are governed by an indenture (the “Indenture”) between us and U.S. Bank National Association, as trustee. The net proceeds from the offering of the Notes were approximately $408.6 million after deducting the underwriting fees and other expenses paid by the Company. The Notes bear interest at a rate of 1.125% per annum, payable semi-annually in arrears on March 1st and September 1st of each year, beginning on March 1, 2021. The Notes will mature on September 1, 2027. In connection with the Notes, we entered into privately negotiated capped call transactions (the “Capped Call Transactions”) with certain counterparties. The Capped Call Transactions are expected to cover the aggregate number of shares of the Company’s common stock that initially underlie the Notes, and are expected to reduce potential dilution to the Company’s common stock upon any conversion of Notes and/or offset any cash payments the Company is required to make in excess of the principal amount of converted Notes. The upper strike price of the Capped Call Transactions is $86.174 per share. The cost of the Capped Call Transactions was