Open Question: Do I need business insurance for a student window washing business?

We would be using telescopic poles, so there would be no possible ladder accidents. I asked about business insurance at some insurance companies and they said that they don't give insurance for a period less than one year (the business will run only two months, July and August). So, do I need business insurance? Will it be difficult to get customers without it? What would happen if we caused property damage by accident?