Use a 15mm spanner, longer the better but regular is fine. Refit the other side and support it and follow MichaelW2 advice, if that fails use a hammer. Both pedals undo turning towards the rear of the bike.I have one of those spanners with adjustable width I am definitely doing it the right way.
If that doesn't work, buy some penetrating oil - PlusGas, for example.Common issue. Give it a spray with WD40 and try again in the morning. Otherwise you might need a longer pedal spanner, something for more leverage, or more brute force Involving a hammer!
They can be useful when you have a number of lightly loaded bolts/nuts to undo/hold. But, for the heavier loaded bolts/nuts, buy the best correct sized spanners you can.Adjustable spanners are the work of belzebub, and are perfectly designed to round the flats off any bolt, get a 15 mm spanner that’s right for the job, then follow the advice above, particularly with grease on the new pedal threads.
OP is a student, and doesn't need another £5 plus expenditure.Get sone RP90 spray designed to penetrate and make disengaging such things easy. Just dont get it on your fingers and be careful what you spray it on or you wont be able to pick things up or hold anything!
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