how to create generic array for entities

PHP Code:
class SchoolController extends BaseController
{
    const SCHOOL_PK_LIST = ['School1' => 1, 'School2' => 2];
    
    public function index(): string
    {
        $school_code_from_login = 'School1';
        
        $model = new SchoolModel();
        $school = $model->find(self::SCHOOL_PK_LIST[$school_code_from_login]);
        dd($school);                  // point A

        $schoolDomain = new SchoolDomain($school);
    }
}

class SchoolDomain
{
    public function __construct(
        SchoolEntity $schoolEntity
    ) {
        $this->schoolEntity = $schoolEntity;

        // Perhaps there are three semesters in American schools.
        $this->periods = $this->getPeriods($schoolEntity->id);

        dd($this->getCurrentPeriod());      // point C
    }
    
    /**
    * @param int $school_id
    * @return PeriodEntity[]|null
    */
    private function getPeriods(int $school_id): ?PeriodEntity
    {
        $model = service('periodModel');
        dd($model->where('school_id', $school_id)->findAll());              // point B
        return $model->where('school_id', $school_id)->findAll();
    }

    /**
    * @return PeriodEntity|null
    */
    public function getCurrentPeriod(): ?PeriodEntity
    {
        // TODO: return a period entity
        $today = new DateTime();
        foreach ($this->periods as $period) {
            $from = $period->from_date;
            $to = $period->to_date;
            
            if ($from <= $today and $today <= $to) {
                return $period;
            }
        }
        
        return null;
    }
}