How to convince my mom i dont smoke?

im 15 and i smoke on occasion, and i always told my mom i would never go near a cigarette and she beleived me,and at the time i was serious, but i began smoking socialy, and now my teacher in school tells me mom i smoke and at first i thought i dodged that claim, but now ive been spotted smoking with my mates by one of my moms freinds and she told my mom. but the thing is ive gone from smoking 6+ day to two a day then 1 a day then only on weekend social meetings. Now ive stopped completely but after my moms friend saw me smoking on occasion that has added to the already high suspecious mother than i smoke all the time reguarly. but i have stopped completely now so how do i tell my mum i do not smoke at all without telling her i done it regualy before? im willing to tell her i done it a few times but that was all? please help me becoz its really worrying and please no time wasters just your honest opinion and helpfull advice...:)