If i go regularly to the gym for summer vacation then stop would that make me gain weight?

im 15 and i am 5 ft 3" and i weight 56.5 kilos considering i was wearing heavy sneakers and clothes, i want to lose weight desperately because i can see that i have some cellulite, i am on a table tennis team in a sporting club and it's very exhausting and burn a lot of calories but it mainly works on your arms not your thighs so i now started cutting all the crap snacks and eating a healthy breakfast and stopped eating too much meat and i sometimes go with my mom to the sporting club at 7 am and walk 4 kilometer fast walking and then i go to the gym for one hour of resistance training but i don't put heavy weights and i go for 2 times per week and my friends keep telling me that if i stop during school i will gain even more weight that what i lost, is this a myth?